Checkpoint abuse should be easy to fix. You should save the checkpoint of the last section you beat; not the last section you died on. That way people cant just join, die, and leave. They would have to stick around and beat that section. If they left after that, then they would already be past the checkpoint they wanted to steal. Totally fixes the problem.
I would say, the checkpoint means nothing to you, if catch from someone, unless you clear the other levels as well (so even if you beat round 4, if you did not beat round 1 2 or 3, then you will start at round one when you initiate the game... just one more step, do not disagree wit someone joining another whos completed all those checkpoints for final fight, just think when you start, should only skip you first completed levels... (maybe be able to skip over round 4, to round fiv if completed round four with someone else... maybe.... though you could always leave, then re-enter prison to skip over if have completed round 4 before the rest =p)