Not in an iterated game. If my team agrees we'll never checkmate before turn 5, the game is the same except we start the actual game on turn 5 with a big score advantage compared to everyone else.

You can leave at any time by breaking the rule, but then you will be playing with other people who say checkmate immediately, and that would be much worse.

Being prosocial is in fact a stable equilibrium. As prophecized by gestures broadly at everything.