EDF, first I want to say that you are incredibly strong, you ability to disengage and distance from the very start of this process is enviable. I have no idea how you kept your knowledge of the EA to yourself for so long, but I envy your strength.
I just went back and read all of your sitch, well skimmed through some of it, but I have a couple of questions. I read something where you mentioned setting a boundary of no open relationship, but I don't recall what the consequence attached was. Also, is your WW still in the MBR?
Just my thoughts for what it's worth, but it's been two months since you discovered the A and it's still ongoing. You say that it's petering out, so have you considered putting your foot down and seeing if you can get her to snap out of the fog and end the A?
This might be a good time to tell her that you understand her needing time to work through her situation, but you've given her time and she hasn't ended the A. That you refuse to be in or part of an open marriage and that if she doesn't end the A you will move forward with separation and divorce process. You can state that you are separating your finances and she can move forward with purchasing a new car on her own, and tell her you will not share the MBR with a cheater if she's still in there.
From what I've read she doesn't seem that far out there, so this may be enough to jolt her back to reality, and if not, how long are you willing to wait before A escalates or a new one begins anyway?
I think everyone agrees that the MR can't move forward until the A ends, and it just seems to me that letting it drag on for too long may make things worse in the long run.
M - 9 1/2 years 5/5/16 - Bomb drop - 3 week EA 10/31/16 - We sold house 01/10/18 - D Finalized