Before I say anything let me say that most of what you describe regarding your H is 100% completely normal.
He wants to deal with this on his own as much as he can because he doesn't feel safe with you right now. He also seems to be trying to out tough this. He is confusing stubbornness with strength.
He isn't asking you questions to see if you are telling the truth. He is asking because he is trying to process this. Again, 100% normal and even expected. Don't think about it too much just answer honestly.
The difference id the bedroom. Tread lightly. There are many ways to answer questions related to this. Being honest versus brutally honest is required. The worst path is to avoid them. He has made up the worst case scenario in his head. He is also letting his gender assume for him that you are a man and you would cheat on him for the same reasons a man would. Very common.
Treat it like a factual account, but try to keep the commentary to a minimum. We did x, y and Z is much better than: he did x better than you, He did Y which you never do and z you were about the same, but since it "naughty" it seemed better. Not answering is far worse than anything.
Nightmares are common too. Looking to trach OM a lesson is also common. He is trying to protect his love for you by making it the evil OM seduced his innocent wife. You need to tell him that OM is irrelevant. OM does not mean anything to you or him.
Sadly, OM could have been anyone. You made the commitment to him and therefore his issue is with you. I know it is scary for you to say that, but outside of an IC no one is going to help him move past that hurdle.
Effort is important right now. Actions. Words mean very little. Try your best to be loving to him and nice to him even if he responds in anger, sadness or detachment. Pretend like you are trying to impress him.
When he says you are just doing it because you got caught, correct him that it is because you love him.
Also gently correct him when he says that you just wanted to sleep with OM. It wasn't about that. It was about the attention he gave you. Also offer that you don't know why, but you are going to work very hard to give him that answer.
Amends are important. Especially with BHs it tends to help when you share how much shame, guilt, sadness you have over making this choice. Offering up anything to help him heal, including agreeing with a D, if it will make him feel better.
Right now he is approaching this from you are going to do whatever you want to save the M. That is true right ? Change that to do whatever it takes to help him heal, even if the M has to end.
I know that is scary, but you have to remove any inkling of ulterior motives. He will see them in everything you do. Everything you do is suspect and he will think about what you have to gain from it. Remove those if you want to reach him. Obviously if you are only trying to save the M, don't say that. No lies.
Look. I am a guy-guy. I played NCAA division football, I change my own oil, I hunt and speak very little. My W still cheated on me. There was nothing I could have done differently in the circumstances to change this outcome.
Also even though it was difficult I went to IC. It was a great thing. It helped me more than anything.
My M may not make it, but I am going to be OK no matter the outcome. THAT is why you go to IC (Tell him that).
SI has an active BH community and our own forum. Invite him to join, read, post. Sometimes it helps to get a guys take from guys that have been there before.
Take care and keep going. You only lose if you give up. That goes for both you and your husband.