1.Can we make stops on our way?
Yep, why not? That is the usual logic for any domestic tickets in that part of Europe. As far as you travel during the validity period, you can make as many stops as you want. We didn't do such thing particularly in Slovenia, but in neighboring to Slovenia - Hungary and Croatia it worked perfectly and without any problems. The only inconvenience is that you might be obliged to get special stamp on your ticket (from service center or other station official) in case you make a stop.
Can we simply buy only one return ticket Zidani Most -Maribor -Zidani Most ?
Yes, it will be valid for 15 days
Can we buy an IC return ticket and use LP/RG trains with this ticket?
Yes again, company operator is the same, even tariff is the same. The difference just in special supplement for IC trains (
http://www.railfaneurope.net/pix/si/ticket/sz_zuschlag.jpg), but no problem to use it on 'lower rank' trains.
Or buy ICS return ticket and use IC, LP trains also? (this can be complicated due to compulsory reservation in ICS)
How compulsory reservation is a problem here? It's not necessarily printed on the same ticket sheet, so there should be no problem buying just ticket (or ticket+one reservation) and again should be no problem using it on LP/RG
p.s. obviously better to recheck everything with SZ once you there, but we would be really surprised in case something of the above is wrong, since it all looks totally logic.