One reason for playing multiplayer scored rounds is that you get XP. If you do that and people quit after maybe an hours playing, it is easy to get annoyed. It is extremely rare that there is no dropouts and the majority of the games that I have played with 4 players ends the same: I finish them alone and get XP only for a single player round. It is also very common that a person leaving freeze the game so everyone have to quit.
I would suggest something like this:
- If more than half the game is played as multi, XP is awarded as if it was multi
- If someone quits, they get a warning flag. A visible warning flag. To get rid of the warning they need to finish, say, 5 multi games
- If someone that has a warning quit again, then they are punished by removal of XP. Maybe -1000XP is reasonable.
I think that would go a long way to fix the situation.