How about a mediator? Treat it like a regular custody situation only with a shorter lifespan. And, yes I'm serious. Have a neutral third party (preferably one with some counseling training to be at least sensitive to the emotional dynamics) can be really helpful in her seeing what is reasonable & what she wants are not the same thing.

And, you've made lots of progress since that ill-fated MC session.

Me: 50 W:43
S6, S3
M: 12 yrs. T: 17
M is bad & Not happy Bomb Mar '14
S 5 Feb '15
D Bomb 13 Apr '15 (but "no hurry")
DB Coach May '15
Wants proceed on D Aug '15
Starting 1-on-1 negotiations Sept '15