I've struggled to understand this thread all along, but since it seems both the OP and her DF want to see the relatives, and it seems MIL grasps that she overstepped and the shower aspects won't be happening, I really don't see why the OP and her DF can't go to the reunion.
Because we have no proof the MIL has actually grasped this. She *says* she has, but she's also lied about it before, including saying she hadn't sent invitations yet (when she had) and that she had sent invitations only to LordL's side of the family (when she had send them to LadyL's family too). So far the only consistent thing is that MIL will verbally back off when confronted, but still do whatever the eHell she wants anyways, lie about it until she's caught, and not be at all sorry about any of it.
So if they back down and go, I would bet money on a shower happening anyways.
As with PP's, I think going and trying to have a stealth reunion after the main event without MIL invited is an extraordinarily bad idea - even worse than giving up and letting MIL host the shower. What I can easily see happening....
- MIL hijacks it, and shows with the rest of the guests and a shower prepared (or a set of hysterical grievances). She has the whole reunion to get everyone else involved, after all, and you won't be there without your side of the story.
- The rest of the family gets actively dragged into the fight.
- You end up with a screaming fight at your stealth reunion, and shunned by most of the family, who then boycott your wedding.
From previous posts, it sounds like the family in general has issues, so I wouldn't depend on people seeing your point of view, or taking your side over your MIL's, or even not bringing MIL with them to your party. And if you throw an after party for a reunion you've boycotted, you will lose the moral high ground.
Setting boundaries often has a cost. The cost of this boundary is staying away from the reunion completely. It sucks in the short term, but will be worth it in the long term.