kiwigal, hi!

I can understand much of how you feel, I have similar challenges with a uBPD MIL. I'd also love to hear more about your husband's role.
How do I hold a place of dignity while holding to my core values of honouring my father in law?
I connect with your feelings here. Changing a dynamic that's existed for 16 years will take time. I'll gladly try to help you think through this. You've identified three options:
1. Not going and becoming the scapegoat.
2. Speaking up and feeling invalidated.
3. Going and risking being perceived as falling in line.
Did I capture that correctly? It could be that these are your only options, but just in case, let's stretch these possibilities.
1. Are there any reasonable scheduling conflicts that might conveniently prevent you from attending with a very sincere apology and commitment to see them at a future family event?
2. If you speak up, what do you think about developing a simple statement that will communicate a boundary, but will minimize the chances of either invalidation or conflict?
3. If you attend, is it an option to attend and stay in grey rock mode? Is there a way to join them, but only for the beginning because you also are responsible for (fill in the blank) that day?
I'm interested in hearing more.
pj