I understand you feel hurt and betrayed but it seems no matter what, you look at everything your ex does as a negative.
Yes, it sucks the fish died but sometimes fish die. My neighbor is a fish freak. He has a gorgeous aquarium and he loves and cares for his fish like crazy. He has had fish die for no reason after a few weeks from time to time and it was not due to neglect. Maybe your ex did neglect the fish but maybe not.
And maybe the co-parenting seminar will be good. You and your ex might learn something that could really help you work together much better. It could turn out to be really helpful. Why not go in to a required seminar with a more positive outlook?
IMO your C should not be making comments about your ex and her motivations if she is not present. He should be helping you develop tools to deal with what comes your way (surprise custody papers). It kind of seems when he makes comments like he does (your ex sucker punching you or telling you your behavior when you stormed over there was understandable) he sort of keeps the negativity going. You or your C can't control what your ex does or does not do. What the both of you can work on is how you react and handle it.
