NTA

If they wanted it, I would go through with it no questions asked and told them as much. But they're saying it's my decision.

My gut would say to do what your partner wants. It sounds like they're punting to you, but you nonetheless feel they'd rather not go through with it. I'd trust that feeling and decline to participate in the wedding.

If your partner is being 'painfully supportive' it may be because they don't want to make a bad impression on your family, knowing that the invited relatives will get a spun narrative as to what happened and might not understand the fall out. But that's part of your mother's calculations, you've been manipulated into a bad decision.

Declining the wedding may put you on the wrong side of the family, but you'd be setting the tone of your whole marriage that 1) you'll always put your spouse first, 2) you can set reasonable boundaries with your family to protect your marriage 3) you don't care what other people think and you won't be manipulated. Don't be a pushover right from the start.