I agree, reluctantly, with your NGS group. By not going, you show her what it means to be separated and/or divorced. It's tough on S14, but best option overall. You would have to explain to your son ahead of time. Also, if you have a good or cordial relationship with your in-laws (and your children's grandparents no matter what happens), then you might contact them and say, "Hi, I wanted to let you know I'm not going to attend S14's b-day dinner with you. I would like to go, but my presence will just create a tense environment for everybody and ruin his meal."
Of course it seems I'm not following my own advice because my D14 has her birthday on Wednesday and we were all discussing last night where to go for dinner. The only difference is my W and I are not fighting in the way you describe. Now I'm going to think over whether I should politely decline to go out with the family and the tradeoffs involved.
About your Sunday crazies discussion, I think others will have more to say. It looks like when she pushes your buttons you push her's back. Don't.
You could also read the thread on boundaries and maybe prepare better ways to respond.
