You sure you didn't marry my one of my X's four sisters??
Has your W started shopping for a Harley yet?
Seriously, you have as clear a perspective on all this as anyone possibly could. Understand that the thoughts you have of drawing a line in the sand are reasonable. Maybe not for the reasons some think.
If she is so far gone into crisis that she will Never accept life as positive, she may have to go her own way and live the negatives a while. You don't want her to stay and always live in doubt and resentment. You would soon see that was all that was left, and you wouldn't stick with it.
Keep up the fight, you are doing great. But so much is up to her. She has to make some decisions that she just may not be able to make right now.
My main thought here though is this ... if you have to let her go, if she just has to leave, that does not mean there must be a D. It just means that things aren't going to be fixed together in the home at this moment. When something isn't working, it may be time to try something else, but do it with as positive a spin as you can if it comes to that. We did not create separate spaces as friends. It may have turned out different if we had. Don't know. Just a thought for you.