A lot of this is spot on and I am considering just what you said, and no she is not a star player in fact she has sat more than anyone on the team due to performance. HC really has a hard time being the bad guy (I'm speculating), and I think AC is potentially playing both sides or easily swayed.
I talked to my daughter and she thinks I wasn't mean at all, also she doesn't want to stay if this kid stays.
My plan is to inform HC that this will be her last chance or we are gone.
This is a total dumpster fire.. You've been given the same advice over and over about leaving STAT... Part of the reason for leaving is it will give YOU time to reflect and improve as a coach. Remember, coaching the players is only part (sometimes small) of the gig.
Also, really reflect on what the role of an AC is... If you don't have the backing 100% of the HC (no if's and's or but's) then you've really got to analyze your position and how you got there..