Holy crap your FMIL is nuttty!!! I feel sorry for her that she is that controlling. I think the best way to deal with it is to let it come from her son. He really needs to tell her to stop. This is your wedding and not hers to change the date. I would be raging if I were you as well and you have every right to be mad. I give you a TON of credit for being as graceful as you have been (I'd probably be in jail right now). In his own mind I'm sure he put you at ease last night and all is fine with you. For these men, we really have to literally tell them every little detail or they just don't get it.

Good luck to you!
Andrea