Hi Mother of Pearl. I'm glad to see your WH is willing to work on this with you. I understand your fears, but truth is if he is not going to commit 100% he won't with you there or not there. I think at this point you should respect what he is asking of you and simple let him know that you hope that he can be very open and honest with you during this time as well. Let him know that you are there for him and will help him through his withdrawal as much as you can so that the two of you can begin to move forward in repairing your marriage.
I think it's wonderful that he is agreeing to MC. Take that as a good sign.
This is hard for you, and I understand that. However, you have made the choice to work with your marriage, and that work is just now beginning. The first step is trying to understand what he is feeling with cutting off this relationship he had with the OP. This will probably be one of the hardest parts for you, as you will want to try to be there for him to hold him up, but it will also be very painful for you. Talk to him about this. Let him know that you want to be there for him. That you want to help him through it. Let him know straight forward that it will hurt you, but that you are willing to go down that road together if he will let you and stay open and honest with you.
As a WS he may be inclined to try to *protect* your future feelings by not disclosing more. Let him know it is safe, even if it hurts you or upsets you. Let him know that while it will undoubtly hurt you, the two of you will be able to grow stronger because of it being placed in the open, and because you will be working through it all as a team.
This will not be easy for him either. He has to face a lot of deamons within himself. He has to look in the mirror and see the ugly person he had become. At times he may not want to lean on you out of fear of exposing more of his ugliness to you. Simply be there, and let him know that he can take his time too.
You guys have a long hard road ahead of you. It will take time and commitment, but you are started in the right direction. The fact that he let you listen to his conversation is a big step. The fact that he is willing to change shifts to avoid her, says a lot. Hang in there, while it may seem dreadful at times, there is a light at the end of this tunnel, and it is well worth it in the end.
Know this - it's ok to not always have the answers. I'm not sure how much this will help, but I hope it does some. I think the fact that you guys will be starting MC is a perfect first step in the right direction.
Allow him his space, and continue your plan A. Make the most of every minute you two share together, and I bet before you know it he will ask you to come home and be by his side there too.
I wish you all the best.