Quote: She reiterated that what she wanted from me was a promise that "for better or worse" I would not leave the marriage.
That is crap, and I'm not sure you even should have answered it. She wants an absolute promise from you, but she won't give you anything solid in return.
Quote: After I told her that I would not leave the marriage, she said, "I'm going to have to let that sit with me for awhile."
I don't even know what that means.
Me either. I hope the MC can get through to her that she has to give you something more useful than that. That's probably why she (the MC) said not to discuss these things on your own. Still, that sounds like a good conversation (on your end, anyway).
Good luck, it sounds like things are rolling forward, slowly but surely....