I think your D's reactions are very very normal. I think isolating herself in the room to think things over, realise what just happened, come to terms with your devastation which she clearly sees etc etc is a healthy reaction. And as you said, the more invested she was in your family the greater the shock is for her as well. My 9 yr old D, wont allow anyone to even say a bad thing as a joke for her father. If she had to face such a realization I am sure she would be devastated.
Discussion and questions (about whether other people's families are better off divorced), IMHO, can be done after the initial shock has subsided and when she is ready to participate. And it sounds like she is getting there.
I can imagine the stress you are dealing with, the affair, the OW, your kid's emotions. Especially the kids can play a big role in what happens in the future so I believe it's a good thing you are dealing with this and are active about it and strong.
It is amazing he signed you in for Retro already. There is thread in piecing from my friend Sara who is actively involved in Retro programs that you may find helpful.
