This seems to be the best idea though I realiise it can't be used on IC trains.
Yes, you absolutely right, for a group of 4 people Mecklenburg-Vorpommern-Ticket (Mecklenburg-West-Pommerania-Ticket) is the best option. And don't worry about IC or ICE trains, for both legs from Warnemünde to Rostock and from Rostock to Schwerin those aren't really an option, because it's all about S-bahn (urban) and Regional trains here. And anyway since it's only hour and a half journey from Warnemünde to Schwerin you'll hardly regret not having a possibility to use IC/ICE trains
Does this pass cover all 4 of us for our outward and return journey?
Yep, 34 euro (price of Mecklenburg-Vorpommern-Ticket for 4 passengers) will cover any journeys in Mecklenburg-Vorpommern during the whole day (from 9am on weekdays or from 0am on weekend and holidays)
I believe we have to change trains at Rostock - can we get on and off any time and return at any time? Why I ask is that it the website asks for a departure and return time.
Yes, you can easily get off or board any train (as far as it is REX, RB, RE or S-Bahn) and make any stopovers you like. Regarding DB website that asks for departure time there is a small, but important nuance. If you try to buy it using DB timetable search, then it will be sold with link to specific trains you choose, thus you in advance tie yourself to only those trains. But if you purchase same Mecklenburg-Vorpommern-Ticket at the station in Warnemünde, then it won't be linked to any specific train, thus you will be free to go and stop anywhere. Same things happens once you purchase your Mecklenburg-Vorpommern-Ticket not from timetable page, but directly in
TicketShop. Though we would advise you not to think about it in advance and don't bother with online purchase. Buy it already in Warnemünde from DB ticket machine and don't worry
Can it be booked online and can I print the ticket?
Yes, you can book it via the link we've provided above, but again what's the point? You can buy same ticket for the same price already in Warnemünde before departure
Is there any advantage in booking early? (we are not travelling until May 2013)
No, at least at the moment fares for all the Lander Tickets like Mecklenburg-Vorpommern-Ticket are the same no matter when you purchase. The only difference is that if you buy them from the ticket counter you'll be charged additional 2 euro, that is why better to buy via DB ticket machines and pay 34 euro instead of 36 euro at the counter
If, for some reason, we are delayed and I have to change the day - is that possible?
Well, this is one of the reasons why it doesn't really make sense to buy Mecklenburg-Vorpommern-Ticket online & in advance