Personally, I would go against the dating thing until she makes up her mind what she wants to do and files the paperwork.
The person in MLC will do things totally against their moral and ethical character. If it's against your beliefs, then you need to tell her. I think that if you don't do that and compromise what your ethics and morals are, then you may end up resenting her later even after you reconcile.
Someone has to be the moral compass in the relationship and unfortunately that falls to you. Let me put it this way. Who do you want your kids to model their behavior after in terms of M. You or your W?
Keep up with the positive interactions with her, but not at the expense of your own morals. You can't stop her from doing what she wants, and likewise she can't stop you.
I think your MC is very pro-M which is a good thing and not necessarily against DBing. If you two are uncomfortable about the C harping about the age of the children, then ask her to not focus on that but to discuss another area. You want someone pro-M on your side. But of course a little more toned down.
Because of the MLC-ers selfish nature right now, you need to change all of the negatives that they have into positives. And structure your discussions with her so that it seems like it benefits her when in fact it benefits you.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.