A big bomb today, indeed. But it changes nothing. The day is long.
I agree that you should not agree to back-date the separation. Time is your best friend right now. It's the ultimate healer and it has a way of changing peoples' minds.
I have a feeling that you putting your foot down on a couple issues -- notably the separation back-dating crap -- is going to take your W aback. Again, I agree with jarhead that you're probably gonna feel some backlash from her. But making that stance is gonna do two things for you: A) It'll buy you some time, and B) It'll be a 180 for you. And I think, while it's gonna p!ss your wife off, it's also (perhaps subconsciously) gonna impress her. It'll be your first step in becoming the man who's more of a man than OM. I think your W wants a man. Not a doormat. You've been a doormat for too long.