Yoda...
It is a healthy decision for you to set boundaries on your Mother and her actions.
Jmo, but you have put up with her disrespect for a very long time.
I believe your refusal to keep pretending is a giant step in taking care of yourself...and realizing that you are a worthwhile human being.
Akin to saying, "I will no longer allow you to treat me this way."

You are being fair, Yoda, in what you plan on doing tomorrow.
If by any chance he has not left yet; leave. Don't explain...walk out of there with your head held high.
Be consistent and she will realize you are not coming to family events when your abuser is there. An alternate plan like you suggested re coming later for leftovers is a valid one.

Best wishes...

Catherine