I don't think I can handle it if he becomes an active joint parent with my W and is making daily and weekly decisions and communicating with her often.
Exactly, b/c she is not over him. You would be supporting her and the baby, but he would have her heart. At least till she got fed up with him. But you saw how fast she wanted him there at the doctor's office.....and that was after he dumped her! So, it could continue a long time.
I would get in no hurry to tell her about this. Give yourself a few more days. Also, see what her next move is going to be. Does she take for granted the two of you have reconciled or is she waiting for your answer?
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!