This really is gobsmacking. I am getting so irritated on your behalf Tidy i feel like flying half way round the world and gate crashing their wedding and opening a can of whoopass on this fruit cake (speaking of cake i hope this doesn't go the same way as the last minute cake bridezilla thread - utter madness and getting family involved too).
If it was me i would have to reply with an unemotional run down of the facts. I would also point out that no one should 'rely' on anyone they haven't already agreed something with. How fucking presumptuous 'i know, i wont invite Tidy (and 20 others) to save cash, and because they've saved the date then i know they are free to run around helping behind the scenes of my speshul day. I mean WHO wouldn't be HONOURED to do that' 
And then get upset. NOT because she has pissed all over your friendship, oh no, but because YOU are not helpy welping the pwincess bwide out. She's crazy in the coconut.
I'M FURIOUS FOR YOU