My H and I were the same way. He was always the pursuer. A 180 for me would have been to pursue him...BUT that is not what they want right now. They are past that. I don't think you should go no contact, but maybe just limited contact. Do not bring up the R or OW. Trust me, I know it is hard...and sometimes impossible. Do not bring up divorce. I think by me constantly bringing it up, it finally drove him to file. Just don't mention it.
How does he know OW if she is in England? I think your H has been through a lot. The way he is ruining relationships and cutting people out of his life points to PTSD. I think my H was is and still is PTSD. He says he is ok, but I know better and don't want to rush back into anything. I am sure your H seems like a different person. It is so hard to watch them self destruct. If he is doing this "cutting" self mutilation stuff he needs help quick...even if he gets madder at you.
I was going to suggest maybe you contact his commander and tell him that you are worried about him. They can require counseling. I am not sure if your H is in the reserves, but that is what it sounds like and I don't know how that works as much. My H is in the Air Force. My H's boss noticed his change at work and made him go to counseling. If this is an option for you I would make sure your H didn't know you suggested it.
The best thing you can do right now is be patient. You also need to work on yourself. Try to detach and GAL. Do this for you, not for him...but he will notice. Don't try to make him notice. I think they pay attention more than we give them credit for. Another thing, your imagination is your worst enemy. Don't try to think about what he is saying and doing with OW. I am sure it is not what you have in your mind. He is messed up right now and couldn't be there for anyone. Sounds like she is messed up too. Eventually he will see he needs something stable in his life and she is not stable. Right now she is a band-aid on his problems. She doesn't ask him questions, she doesn't know he is different. For now, just give him space and work on yourself.
Are you two still in the same town? Do you have any reasons to contact him? We have no kids so I didn't...although I did call about bills or finances every now and then. If you talk to him, do not bring up R. Trust me, you will only feel worse.