Just in case I can't get the Belgrade - Bar special, do you know how much a return is?

Full fare Belgrade-Bar-Belgrade bought in Serbia (or Montenegro) costs 42 euro for the ticket (+potential reservations), and standard tickets definitely can be book via Serbian Railways (ZS) Ticket Shop, that's 100%. However taking into consideration that "Montenegro Spezial" Budapest-Bar-Budapest is only 66 euro, it turns out a euro cheaper than combination of 42 euro + 25 euro for the Budapest-Belgrade return

And there isn't a couchette/sleeper from BUDAPEST to BAR directly (if I take the evening departure)... but from BP to Belgrade is doable.

Yep, you can get sleeper/couchette only for the #341 Budapest-Belgrade (Bc6 - 8 euro, Bc4 - 10 euro, T3 - 12 euro & double is 18 euro, all bookable via MAV website), with daytime #431 Belgrade-Bar it's just seating carriages.

I'll try to persuade my companion...

As of one the persuasion arguments you can advertise evening departure from Bratislava by the fact that you get to travel Belgrade-Bar route during the day. Belgrade-Bar railway is one the most scenic rail journeys in Europe, so for those who never traveled there during the day it's definitely a must see thing 

[Berlin to Stralsund]

If I get lucky and order in advance the needed tickets AND get the cheapest tickets, point to point is the way to go.

Well, taking into consideration your itinerary it seems that purchase in advance & discounted tickets aren't always the best solutions for you, so you don't really need to tie yourself in all the journeys, because you easily can still have some flexibility even without Eurail pass. But let's look step by step:
Berlin to Stralsund: here you not necessarily need to "hunt" for discounted tickets, because German Railways (DB) offer pretty good local train passes. In case of this journey ones to consider are Schönes-Wochenende-Ticket (for weekends) or Quer-durchs-Land-Ticket (for weekdays). Both passes allow unlimited journeys during one day all over Germany for groups up to 5 passengers, and best thing about both passes is that they can be bought even before departure, so such passes are perfect for day trips like Berlin-Stralsund-Berlin. Quer-durchs-Land-Ticket for two passengers cost just 50 euro, while Schönes-Wochenende-Ticket even cheaper - 42 euro. As result both of you can travel Berlin-Stralsund-Berlin for not more than 50 euro and without any need to buy anything in advance. The only minor inconvenience is that such passes are not valid for IC trains, so you'll have to use slight slower regional trains (although in case of Berlin-Stralsund difference is just extra 15-20 minutes). More info on Quer-durchs-Land-Ticket & Schönes-Wochenende-Ticket you can find on our Germany page
Berlin to Potsdam: Potsdam is really close to Berlin, so even full fare standard tickets are very cheap (3,1 euro one-way from Berlin Hbf to Potsdam Hbf), so definitely spending Eurail pass day on daytrip to Potsdam is useless, just buy regular ticket before departure and don't worry about the price
Berlin to Munich (with CNL train): while here because of a long journey indeed purchase in advance of the discounted ticket is a reasonable choice, but I guess you won't need a lot of flexibility here, thus buying Berlin-Munich ticket in advance isn't that big of an issue
Munich to Dachau: and again, as with Berlin to Potsdam, Dachau is not far from to Munich (just 11-21 minutes by train), so standard full fare ticket is your choice. Plus here you can get MVV (Munich Transport and Tariff Association) which will cover not only journey to Dachau, but also public transport. Partner Day Ticket from MVV cost 13,6 € and both of you can ride as much as you want in Munich and around
Munich to Salzburg: again, local German pass is your solution here. In this case best choice is so called Bavaria Ticket (Bayern-Ticket), idea is the same as with Quer-durchs-Land-Ticket & Schönes-Wochenende-Ticket, but here it's valid only in Bavaria and some border stations (including Salzburg). So for 26 euro (price of Bavaria Ticket for two passengers) you can travel Munich to Salzburg without any need to buy tickets in advance, show up at the station & buy your Bavaria Ticket via DB ticket machine, that's it
Salzburg to Venice: due to long distance here a good choice is to buy discounted tickets in advance. Btw, we have a step by step guide on how to purchase tickets via Austria Railways (OeBB) website - http://europetrainsguide.com/Advice/How-to-Buy-Train-Tickets/Austria-OeBB.html
Venice to Florence: two relatively cheap options here: a) purchase in advance via TrenItalia or Italo websites discounted tickets for direct high-speed trains (start from only 9 euro) or b) don't bother with in advance purchase, but simply use slower, however much cheaper (if to compare full fares) regional trains, for which standard fare ticket Venice-Florence is just 15,85 euro (and you can buy such ticket even 5 minutes before departure)
Florence to Rome: as in previous case, in order to travel cheaper you can either buy in advance discounted tickets (again start from 9 euro) or opt for slower regional trains (standard full fare is 20,25 euro) and don't think about buying tickets in advance

I wanna take EC 273, which is direct from Bratislava.

Well, it's not really a direct train (since EC #272/273 "Avala" it's just Prague-Belgrade), but two through Czech carriages Prague-Bar (run seasonally), which get reattached in Subotica for Subotica-Bar #1137 "Panonija" train.
And because of all these it gets a bit complicated. The thing is that "Bar Specjal" offer (allowing passengers to travel even for less than 10 euro) you've mentioned is sold only for direct Belgrade to Bar trains, and you can't buy it for Subotica to Bar train. Plus at the same time you also can't really rely on traveling by "Avala" all the way to Belgrade in hope to change there, since "Avala" arrives to Belgrade at 20:52, while last Belgrade-Bar train (#433) leaves Serbian capital 20:10. So judging from the above your choices are either to stay the night in Belgrade and continue to Montenegro next morning or forget about cheap "Bar Specjal" tickets and just go with Subotica-Bar #1137 "Panonija" train and regular tickets, which will allow you to be in Bar already next morning.

Can I buy Budapest to Belgrade special from Hungarian conductor? Can they be done online?

Hungarian conductors doesn't sell such international tickets, however you can purchase it online via MAV website and simply print it out via MAV ticket machine at Keleti station during half an hour stop of EC 273 in Budapest

I guess it's quite expensive to buy it as one ticket from Slovak rail, right?

A few years ago citystar ZSSK-ZS/ZCG was 98 euro, now it should be slightly more than 100 euro, so... kind of doesn't really make sense to buy ticket directly from Slovakia to Montenegro

Oh, and just seen this "Montenegro: 'Montenegro Spezial' Budapest-Bar return for 66 euro". Maybe this can be bought online

Unfortunately nope, MAV website doesn't offer a possibility to purchase it. Again, due to rather long stop in Budapest it's possible to try and buy such ticket from the international counter/wasteels office at Keleti

And we're prepared to do the journey just sitting, but how about couchette. How much can this cost all the way from Bratislava?

Don't have the exact numbers for Czech couchette/sleepers, but Hungarian ones (carriages Budapest-Bar) cost 9,4 euro (Bc6), 14 euro (Bc4), 15,4 euro (T3) and 23,1 euro for the double. CD fares for couchette/sleepers should be roughly the same or not much more expensive, thus you can take these number as a base

So, taking into consideration all of the above, options regarding ticket/reservations might be as follows:
a) purchase Bratislava-Budapest return ticket (17,5 euro) and, if you want, couchette/sleeper reservations Bratislava-Bar (NB, Prague-Bar carriages can be rather popular and sold out pretty quickly, so if you do decide to use them, then makes sense to think about buying reservations early). Then during the stop at Budapest-Keleti just run and buy additional Budapest-Bar-Budapest ("Montenegro Spezial" ticket)
b) take an earlier EC train to Budapest and have plenty of time to buy your Budapest-Bar-Budapest ticket & even some time for lunch along with walk in Budapest. If you want to use couchette/sleeper from Budapest onwards, then makes sense to buy it in advance while you still in Bratislava (reservations for both Prague-Bar (CD) and Budapest-Bar (MAV) carriages should be available from ZSSK ticket counter)
c) purchase return Bratislava-Kelebia ticket (35 euro) and separately Kelebia-Subotica-Kelebia (3,6 euro when bought from ZSSK counter). While from Subotica to Bar (and back) you can get tickets(+sleeper reservations) via Serbian Railways (ZS) Ticket shop, however again remember that during summer sleepers (and maybe a bit less couchettes) get sold out pretty quickly, so makes sense to think about Subotica-Bar in advance
d) forget about "Avala" and morning start from Bratislava, but opt for the evening departure from Bratislava, then overnight train Budapest-Belgrade (25 euro return ticket) and finally daytime Belgrade-Bar. Here change in Belgrade is safe, since you'll have almost 3 hours between arrival of overnight #341 from Budapest and departure of #431 to Bar. Plus in this case you have a chance to get discounted "Bar Specjal" 9,9 euro ticket (we weren't able to get from ZS ticket shop, maybe you would be more lucky, but if not, then it's possible to email ZS (putnik.info@srbrail.rs) and try to arrange "Bar Specjal" ticket via email)

you cannot use Thalys with that pass.

Arrivans, are you sure? It seems that both Eurail & Interrail are still valid for Thalys, at least Thalys website confirms it

...or in Russia likely also the provodnitsa, is allowed by law to sell these places to someone wanting to travel last-minute. CAN-does not perse mean will.

Indeed, but as far as I remember it's not "provodnitsa", but only "trainmaster" (=head of the train) can officially sell tickets and allocates passengers. And nowadays it happens on extremely rare occasions, so, imho, chances to get in such situation are really low

2.as there is the special 1way fare for just 15/16 eur-pay in HUF for BP-all the way to Beograd, this is also the cheapest way to reach Subotica-so there is no real need to get off there and buy BFP. Stay on to Beograd and have plenty of time.

Well, regular one-way ticket Budapest-Subotica is 14,3 euro (when bought in Hungary or Serbia), so it's actually 70 cents cheaper than Budapest-Belgrade one ))) But to be serious, getting off in Subotica can make sense for those who intend to travel to Montenegro right away without stop in Belgrade, since change from 'Avala' to 'Panonija' is more convenient in Subotica compared to the option of catching the same train in Belgrade, where it will necessary to board it at Novi Beograd station instead of main station

You save hardly 1 eur by using local MAV ticket to border and cross on yourself or via Szeged-but loose plenty of hours. Just realistic thinking.

Agree that in terms of convenience neither Szeged or Kelebia+walking option are optimal, but in cases when someone needs to be in Subotica earlier than arrival of 'Avala' it's more or less the only options

1.What is best train ticket options would suit to our itinerary ? Munich – Amsterdam – Paris

Well, between Munich and Amsterdam there is only one direct train - it is overnight CNL (CityNightLine) dep. 22:50 and arr. to Amsterdam 9:49. However at the same time it is also possible to travel quite easily during the day using high-speed ICE and InterCity trains, in this case at least one change will be required (although in Germany train changes are relatively easy and almost risk free, so nothing to worry about).
With Amsterdam to Paris it's even easier, since high-speed Thalys trains depart every two hours (or every hour during morning/evening peak times), so just slightly more than 3 hour journey and you are in Paris

2.Is Eurorail Pass recommended?

In cases like yours, and especially if you prefer rather fixed travel plans, we usually do not recommend buying Eurail pass, simply because it turns our to be more expensive. Let's just take a look at some number to see what we mean here -->
Great thing about many European rail companies is that they have a variety of tickets prices, as result passengers purchasing their tickets in advance (for instance right now for mid August) can enjoy discounted tariffs and pay much less compared to the normal standard fares. For instance, if you purchase your train tickets now then overnight journey from Munich to Amsterdam with mentioned above CNL train can cost your whole family as low as 168 euro (reclining seats), 218 euro (6 berth couchette compartment) or 383 euro (one three bed sleeper + another two bed sleeper). With daytime ICE+IC trains ticket as well can cost less than 200 euro for all 5
Similar discounted tickets available for Amsterdam to Paris with high-speed Thalys trains and can be as low as 135 euro for all 5 (35 euro per adults passengers and older child and 15 euro each for two younger children).
So at the end the whole itinerary Munich-Amsterdam-Paris can cost you like 300 euro for all 5 (or a bit more if you decide to use overnight train and couchette/sleepers accommodation), thus really doubt any pass (and especially Eurail) can beat this price. However you also need to remember that discounted tickets have some limitations - usually they are valid only for the specific date & train, plus can't be exchanged or refunded, it's a sort of price you pay for the discounted fares, so they might not suit travelers with possibility of plans change or spontaneity.

3.What station will be arrived in each city since we need to arrange hotel to the nearest ?

In Munich it will be main train station - München Hbf (Munich Hbf), in Amstedam - Amsterdam Centraal (=Central station) and in Paris - Gare du Nord (Paris-Nord station)

p.s. You can check timetable and purchase tickets for Munich to Amsterdam journey via official page of German Railways - bahn.de, same things for Amsterdam to Paris can be done via Thalys website (or French Railways website)

Well, answer a lot depends on your itinerary and how many travel days you actually have with your Eurail Select Pass. Generally it's a good idea simply to list all the potential journeys you intend to take and if you can't cover all of them with you pass travel days, then you should check "cheapest"/shortest journeys on the list for which it's will be ok to buy regular full fare standard ticket instead of using travel day from your pass. Majority of standard fares you can find on official page of German Railways (DB) - bahn.de
Regarding issues with seat reservations you more or less have nothing to worry here, since Germany and Benelux countries almost don't have trains with obligatory seat reservations (should be purchased on top of the pass), as result you can just hop on/hop off on almost any train without need to think about reserving a seat. Seat reservations obligatory only for ICE Sprinter trains in Germany and for Thalys/Fyra high-speed trains in Belgium/Netherlands

Easiest would be to fly, Croatian Airlines offer direct flights from Vienna to both Split & Dubrovnik. Although ticket prices aren't very affordable in this case, so might make sense to fly with change and save some money, for instance Serbian JAT airlines offer more or less good fares for flights Vienna-Belgrade-Split/Dubrovnik
If to speak about going by train, then there is not much of a choice - train from Vienna to Zagreb (discounted tickets start from 29 euro) and then another train to Split (ticket is app. 25 euro + need to add price of potential sleeper reservation). Dubrovnik doesn't have rail connection anymore, so you'll either have to take a bus from Split to Dubrnovik or bus from Zagreb to Dubrovnik
Another interesting possibility, but only for those who have time, is to take train from Vienna to Rijeka (at least one change will be required), while from Rijeka you can catch a ferry to Split and Dubrovnik, journey by sea is really nice, though takes longer than going by land

When traveling from Gorizia to Milan you will inevitably change trains at least one (in most cases change station will be Vencie-Mestre). On leg from Gorizia to Venice there is only regional trains without compulsory seat reservation and with the same ticket price no matter when you buy, thus there is absolutely no need to buy ticket in advance. But on leg from Venice to Milan, as was already said in the other topic, you can use either similar regional train (again no need to buy ticket in advance) or opt for high-speed Frecciabianca train, here you can get discounted tickets (start from 9 euro) if you buy in advance via TrenItalia website

Well, last direct EuroCity train from Verona (most of trains to Austria/Germany originate from here) to Innsbruck leaves 17:02 (arr. 20:32), thus in order to catch you'll have to depart from Milan around 15. If it's not possible then you'll have to travel with two changes: Milan Centrale-Verona Porta Nuova (18:05-19:27), then Verona Porta Nuova-Brenner (19:50-22:52) and finally Brenner-Innsburck (23:04-23:44)
On TrenItalia website you can purchase even right now ticket Milan-Brenner, however all the discounted tickets for this connection (for June 15) already sold out, so only standard full fare tickets are left (=42,5 euro). Ticket Brenner-Innsbruck is 7,8 euro when purchase in Brenner (or via Austrian Railways website). On the contrary, if you ok leaving Milan earlier, then you can buy Verona-Innbruck discounted ticket for the last direct EuroCity train (17:02-20:32) for 29 euro, while Milan-Verona (for instance 15:05-16:27) will cost you either 19 euro (discounted purchased in advance) or 21,5 euro (standard full fare)

I have great difficulty in booking these train Budapest to Milan. Can you help me to actually book and buy the tickets?

There is no need to book tickets for Budapest-Zagreb-Ljubljana leg or trains within Slovenia (like Ljubljana-Jesenice-Nova Gorica), first of all because price of tickets is always the same, so it doesn't matter when you buy. Second, all those trains doesn't require seat reservation, which means that they can't be sold out. So you can simply buy all the tickets before departure and don't worry. Same thing applies to trains from Gorizia to Venice and majority of trains form Trieste to Venice.
The only part of your route for which you might want to purchase tickets in advance is Venice-Milan, here with in advance purchase via TrenItalia website you can get tickets for high-speed trains for as low as 9 euro (full fare is 37,5 euro), however for Venice to Milan you can as well use slower regional trains which cost only 18,35 euro (full fare). As result you can actually travel Budapest-Croatia-Slovenia-Milan without buying any tickets in advance

I note also that the child ticket was more expensive than the adult ticket!

This situation usually happens only with discounted adults tickets, many of which sold cheaper than standard fare child tickets. Standard fare adult tickets can't be more expensive than child one, simply because it goes against any logic

Is this all ready to go or do i still have to reserve seats even though i can't at present.

Yep, everything is set. On the leg from Paris to Mannheim it's just a bit different rules, there seat reservation is always "included", because it's compulsory for all the trains. While with trains from Mannheim to Munich seat reservation isn't compulsory (most passengers don't bother with it), so it goes as a sort of extra

Do you have any rough price estimates for the following options?

a) train Athens-Kiato + Greek Railways (OSE) bus to Patras would be roughly 15 euro or something like that, ferry companies as well offer their buses to bring passengers directly from Athens to the ferry terminal in Patras, guess price is more or less the same. Then price for a ferry Patras-Bari/Ancona can be found here (it's just one of the companies), while Jadrolinija Bari-Dubrovnik prices are here, Ancona-Split would be slightly cheaper
b) bus from Athens to Tirana hardly will be more than 40 euro, then another 5 or so euro to Shkoder and another 5-10 to Ulcinj. And something like 10-12 euro Ulcinj to Kotor/Herceg Novi + 10 euro from there to Dubrovnik 
c) Athens to Thessaloniki by train (from 19 euro if purchased in advance), then Thessaloniki to Skopje by bus roughly 20 euro. Price of a ticket from Skopje to Montenegro depends on bus company and final destination, but let's say from 25 (Podgorica) to 35 euro (Kotor/Herceg Novi) + another 10 euro from there to Dubrovnik
d) well, for instance as of now on August 11 Serbian JAT Airways offer Athens-Belgrade-Dubrovnik for 180 euro with overnight stay in Belgrade, on August 12 you can get Athens-Belgrade-Tivat for 160 euro or Athens-Belgrade-Sarajevo for 160 euro on August 11 etc.