I totally relate to the panic and anxiety you are experiencing, especially knowing how calm your A will be. That happens to me constantly - in my head as well as in court.
An abuser gets right through to our most vulnerable places and throws us off balance at the most critical times on purpose. He's attending this event to mess with you.
Your best strategy for now and in the future is to grow healthier and more confident in yourself. Some of this will happen as the natural result of restarting your life and getting as far away from your A mentally and physically as you can.
For the party, keep breathing first of all. Then calmly look at your options. You are being offered the best advice here. I agree. This is a private event in a public place.
Whatever happens is precedent setting. Throwing a curve ball on the A at the last moment would be a great way to get him off guard. You don't want to give him control - whatever you do!
Positive affirmations and blessings your way! And hugs galore!!!