Long story short: Fiancee and I get engaged. Both sets of our parents generously agree to split cost 50/50, give us the $ upfront. The party line from both is "this is a gift, we want you to design the wedding of your dreams, whatever you want is fine by me". Best case scenario, no?
Now one of the four seems to be changing attitudes. [ETA: Took out details for anonymity, just in case]. Let's just say that she has becoming extraordinarily difficult, demanding, is at the point where "you've made enough decisions, the rest is me", wants me (groom) out of wedding planning picture completely (half the money is my parents). I would say at a scale of Momzilla she is at a 9 in terms of difficulty. But don't want to say more out of privacy. Wants to be involved, but then when we ask for her involvement in some things, she says "Oh, I'm not interested in that". But then will just destroy my fiancee on the things that matter to us.
The other three are still very much "whatever choice you make, we are happy with".
I get the etiquette. We need to involve her and make some choices, because she is paying for some of it. And we love her, we want her to have a special day. However, it is not 100% her money, and the others giving money have said with their money we are free to choose whatever we want. We made the mistake of not explicitly breaking down the money from the get-go, however. How do you set the limits this far in the game and assign what she can have input on and what she cannot? Assume that most of the major decisions have been booked (all of which she hates and is constantly negative about, while the other three are wildly supportive).
She was a perfectly delightful and thoughtful woman before this process, and I know she will be afterward. We just need to figure out how to proceed from here, because we cannot keep doing this.