From what I read, you are so worried about what everybody else is thinking, feeling, wanting, etc.
When I try to figure out other people, I get twisted up and confused.
What do you want? That's it. Keep it simple.
If you go, why? What do you want from being there? To see her? To make sure he goes to jail? If you go, you don't have to sit with the ex.
If you don't go, why?
Look at your two choices - make a list of pros/cons... and that will help you decide what's best for you.