I understand where your fiancÚ is coming from.
Given the choice between going to visit a bunch of family members at an organized event, vs visiting them individually, I'd much prefer the former, and it sounds like he does as well.
It has sounded to me throughout this thread that your mil wanted one thing (big party at the reunion), you wanted a totally opposite thing (no party, and now, no reunion). And your future husband has wanted a third thing (toast at the reunion) that is much closer to what his mom wants than what you want.
So it seems like you have drawn boundaries and are arguing with MIL over what the two of you want, but it doesn't seem like you're taking into account what he wants. Given the relative positions, I can see why he feels like youre coming between him and his family - I see it that way too.
My advice would be to let him handle his family interactions. If he wants to go to the reunion and try to have a toast hosted by his brother, let him try. When it comes to things involving your family or friends, then sure, be more assertive about what it is you want. But if its just his family, let him take the lead and do what he likes.