I would tell HC that he is welcome to work on trust and rebuilding their relationship during the off season. But for the meet this weekend, C3 needs to be there because HE has destroyed the trust between him and your daughter. If he throws a fit about C3 being there, then DD scratches from bars, period. I know your daughter will probably have a fit about that (mine would, for sure, and she's the same age as your DD), but IMO, Mama Bear needs to come out. The only way she is going to feel safe and confident flying around on those bars is if C3 is there. I would just tell her that you are sorry the situation has come to this through no fault of hers, but at this point her safety is of the greatest concern to you and you won't let her risk a probable injury because an adult has his head up his butt (okay, you might want to figure out a better way to say that).
But, yeah, at this point, I would be telling both HC and DD, that either C3 comes, or she's scratching bars. A concussion, broken bone, torn tendon, whatever is NOT worth an L7 routine or an L7 AA score. It's just not.