I would also let him navigate his relationship with the kids. You can't make the kids do anything-just like you can't make your h do anything.
In regards to OW, if you are intimate with your h, then you could establish whatever boundary you are willing to enforce. In regards to exposing or talking about OW, I see no benefits. If he says she doesn't exist, you could say "hold up, we both know you are not being honest," Doesn't mean he will be honest and admit it. Do you think telling him you know will make him end it and work on M? It won't.
Let your h own his R with the kids. That's his choice and decision. It gets easier. Hang in there!
3 kids BD 12/15/13 (IDKIILY. ) Rope dropped Cirque du Soleil style D final 9-9-14 "Some people are born on 3rd base and go through life thinking they hit a triple." Barry Switzer