What if the OW is a by-product, and not the cause for your M problems? It doesn't excuse his behavior, I'm just saying that you may be spending a lot of energy on the symptom instead of the cause.
If OW is an automatic deal breaker, and you don't want to know how or why this has happened again.......then tell the C upfront that you need an answer for that one question, and you won't need to spend time on anything else.
If OW is a symptom and he ends things with her..........the core issues still exist. If it were me, I would want the question about OW answered, too. I don't know if you will hear or see anything else until you are convinced there is no other person making the situation worse. I just hope you will let the C direct the discussions in the session. If your H sees you charging at him in anger, his defenses will rise and you may not leave with a satisfied answer or gain any ground.
If there is any type of inappropriate behavior going on between them, it must end before the MR can work in moving forward successfully.
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!