Is your MC one who works with couples in making the M work.....or is he one that tells you to go find whatever makes you happy? That will be a big difference.
As I said, if your W goes, I think it will be for the C to say that a D is the answer to the problem....and then she can tell her family/friends that she was advised by the C to leave you.
She will probably take charge of the floor b/c she'll be loaded for bear. I don't know how surprised your C will be.....he shouldn't be at all. I bet he's been down this road a lot.
I doubt much of anything you say is going to change the outcome,but I do believe you should inform her and the C that you are not going to life in an open M. The idea that that jerk would call you to give a message to her is so utterly disrespectful.....just blows me away the way he talked to you. I would inform her that that better never happen again. You are not the errand boy nor their answering service.
If she doesn't have her things out of the house, then I would give her a deadline and if is still there, pack it it up and sit it outside.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!