Palmtree

As I suspected, your FFIL (future father in law) is really the problem here. You should urge him or your FMIL to join the OCF and its forum so they can get a better grip on understanding that waiting until after reconstructive surgery is no guarantee that FFIL will be able to attend your wedding even if you postpone it a year! You are in a difficult spot here.
Why not let you fiancee read the links on Roger Ebert who did have all the jaw surgery and found out the hard way that sometime the first one doesn't work. Maybe he can share it with his dad. The basic problem is a false assumption that waiting is going to make your FFIL feel better and be more able to deal socially with a wedding. While he may hope for that, it simply does not comport with any experience of surgery I have read about or for that matter undergone myself.
I have a son but cannot understand your FFIL's reluctance. If he were to educate himself, he would urge you to move up the wedding not postpone it. Just my opinion.

Charm