If he were to cave and "allow" me in the wedding party and I were to say no, then it'd be back to being my fault

I just want to say I've been in this situation. It's where you feel like you must say "yes" even Enthusiastically! even when you don't want to because you don't want your friend to lose your support. These things can be confusing, and you can be sure Adam is confusing Belle's emotions (key to manipulation), and her vulnerability/lack of defenses against him is making it harder for you to see clearly as well. But the truth is she doesn't have you support, because you don't support this union. You're at the point where you have to make it clear to her (and yourself), that you will only support her, if she leaves Adam.

Of course Adam will play it off like your ultimatum is childish revenge on him or whatever. You may lose Belle for now; you may lose her forever. There's just nothing else you're in a place to do except wait for her to come to her senses. If you continue giving in/trying to act like you're happy for her and whatnot, be sure Adam will used that against her/you at some point anyway: "well if I'm so terrible, why would I let you be friends with privacyismypolicy even though she hates me and antagonizes me", and as ridiculous as that sounds, it'll make sense to Belle.