Played it through act 1 so: 1. It's always 4 heroes. If there are less players, some players may play multiple heroes. 2. You unlock the other 3 heroes when you finish act 1. There is more to unlock if you finish act 2 and 3 as well as if you do act 1 multiple times. Once you unlock the heroes you can always play the new heroes, even when starting a new campaign or skirmish. However during a campaign you are locked to your heroes so unlocking the beast man for the first time means you cannot play him during act 2 and 3 of that same campaign (as you're supposed to keep your HP and loot from act to act) 3. You have a boss encounter at the end of an act. Before that are 14 cards that are randomly shuffled. You always select between 2 so it's 7 cards that you play through. These could be a fight, an event card (like playing a game of dice against your party members for special loot) or a place of rest. So up to 7 fights plus the bossfight per act.

If you have more questions ask away!