I've got my posts on here which I'm sure mention it and I've been meaning to go back and start logging some of the milestone dates that have happened. In your mind is that sufficient or should I get something documented by WW or family member, etc?
I'm not sure how it all works in CA and community property states so you are likely to get better answers with your own research and your own lawyer.
Filing jointly gets you a bigger refund so even if you had to split it, it might not be that bad. The only thing I suggest is make a deal with your wife and split the money over and above what you'd get individually.
Married separately = $5,000 refund
Joint filing = $13,000 refund
Thus...$8000 more dollars...she gets $4,000 and you get $9000.
Maybe you just agree to put the whole "extra" refund towards a marital debt (which is just like splitting it) and you get what you'd get if you filed separately. Get her to agree to that and FILE THE RETURN. When the check come be quick to follow up with her and get it done. No divorce has yet been filed so you are free to spend the money and do what you want with it in a NON asset accumulating manner.
Act non-chalant about it and seemingly fair. Maybe the law will screw you maybe not...but all's fair in love and war PRIOR TO HER FILING.
As far as documenting the above, even emails to her innocuously mentioning a move out date would be admissable to demonstrate such date. Just slip it in.
"You know honey, since you moved out on 12/7, we seem to be getting along so much better. Wouldn't giving reconciliation a chance be a good idea? At the very least, we can try to have some fun together as we end this relationship and move on (a shot at Plan A time)"
If she fails to dispute your date...it's nearly a fact and tough to dispute with all the other circumstantial evidence you'll likely be able to use to prove it's the truth. She won't contest such date unless you give her a really big window of opportunity in which to lie. Just close the window with some documentation.
Mr. Wondering