Mb, I have to agree with OP here. A marriage is between to people not 3 and if you and your H are going to work on the M, she has got to be out of the picture. I would recommend giving him the time that is going to be needed to grieve the loss of the R with her no matter what it is EA/PA. Don't push too hard b/c you will push him back to her. I think OWH has a right to know, once you let him know, I think I would leave it alone for the time being. There will be a time for setting boundaries soon and you can read up on how to present those boundaries in a non-controlling way.
Something for you to work on is that you need to let go of getting him to admit to PA. This will come out later on when he is ready and your unconditional love for him will allow you to do this. It is hard and so many LBS struggle with this. We want some cathardic cleansing of the soul from the cheating spouse and it just doesn't work that way for most sitches.
You have him at the table willing to work on and talk about the M, do not push him away on a point that ultimately will not help heal the M. It will come with time, there will be things you guys can talk about and somethings that you will not be able to talk about right now. Focus on the things you can agree upon, start there.
Formerly "missherlove"
Me49 XW49 M17 T19 S16 D20
Expose yourself to your deepest fear; after that, fear has no power, and the fear of freedom shrinks and vanishes. You are free.
