Even if she agreed to go to MC I don't see it helping. Shes just too checked out to take it seriously, even if it was just about communication for the kids. She would be on the defense about it being some covert ploy to make the M work.
If you want to do the letter I say go for it also, just understand its purpose. You state your position from a healthy place, no need to do it again after this. You have to let go from this point on and let things play out how ever they play out. You planted the thought in her head on your position, it may never lead to anything but its done.
Accept what is, let go of what was, and have faith in what will be
