Wow this is a tough one. You may not like what I write, but here goes.
I have learned a few things in life. 1) Don't give of my time freely. A favor is a favor- you do it with a generous heart, with no expectations. I know there is some saying that you "shouldn't give more than you can afford" and it rings true here. You are resentful because it wasn't properly acknowledged the way YOU would acknowledge it; but not everyone is going to play by your expectations. (yes, he was a jerk!).
It seems to me you are very angry about the time spent on these invitations, but to be honest, M does have a point that he didn't ask you to do the invitations- you did it for B. (True, he was not nice about it though. I would have been angry too, but taken it as a lesson learned and saw my mistake).
2) I try to stay out of other's relationship issues- it is not your place to judge. If M treats B like crap and you are against the marriage, why did you agree to stand in the wedding in the first place? Or spend so much of your precious time making the invitations? M may well be verbally abusive and critical; it sure sounds like he is.
If you were really concerned about B, I would have a one on one conversation with her and offer your support. But being in a relationship (and former marriage) like that myself, I know until B is READY to hear it, she will be deaf. It takes a lot to break out of a pattern of abuse, and calling her fiance on the carpet is probably going to make her defend him more.
Since you've said your piece, truthfully I would apologize to B and leave it alone. I would also tell her that under the circumstances, you can't attend the wedding.