Oh my gosh, Coly, poor you! I get it, I was there. I have not reached even the bottom of the beginning of mountain that leads to the summit of detachment, but I think I have made a little farther than you because while I was reading your play-by-play, I was almost shouting, NO NO NO!
OK, so in this case, here is what I suggest -- have all the same feelings you have, that part is fine. But do not let them cross your lips! Zip your lips, run out of the area, throw your phone across the room, do whatever you have to do to not let the words get out. You don't need to FEEL detached to act detached. Maybe you are waiting to feel detached to do it.
Here is what it could look like for what you posted --
Get a little booklet to put in your bag. Anything you want to say in response to H, do not say. Write it in the booklet. Do not allow yourself to answer anything he says for at least half a day but preferably a full day. DO NOT TALK ABOUT YOUR FEELINGS OR HIS FEELINGS or ask him any questions about either of those things. Just go to your little booklet and write all the things you want to say. Make deals with yourself to wait until that night if it's morning or the next morning if it's night, before you say anything to him. Try to not respond to him for several days if you can.
When he moves out or does things that makes you crazy, even if he considers an OW, tell yourself, this is part of the process, this is part of the process, there is no way out but through. I trust that this thing he is trying is counterfeit, and he will realize this once he lives through it and experiences it deeply. Say to yourself, "I let him go, I let him go, I let him go," even when you don't.
Reading what you wrote here, I can see that he feels constantly backed into a corner.
You could also try this one which I have done -- Whatever month you are in, say, I will not mention anything about feelings until the middle of the next month. What ends up happening is that when you have a rule like that, you end up making it through the pain and then you don't need to say anything about feelings anymore.
What I do now, which you'll see from my posts, is that I never volunteer anything but I still have trouble not responding when my H sends me long diatribes about how he is feeling. I still want to make him see the truth somehow. So I practice the above with that when I am able. But as far as ever volunteering anything or asking any questions, etc., I have made it through that school and graduated! Believe me, it feels good to have that kind of patience and self control, and you will have more head space to think about things you enjoy. I still fall into the spirals but they stay in my mind, I don't say anything to him about it.
Coly, if you believe in God, there is a much easier way to do all these things -- I don't want to post that here but all the things I do to enable these things, I translated into secular language here. If you visit the site of rejoice ministries, check out how to zip the lips as a start. I actually became a Christian because of my stand, and when I started I just followed some of these things as an exercise, I was kind of atheist and Jewish! But I found it really helpful and easier to follow those teachings through that lens. Eventually following them worked on me in another way but that's another story! You could also try doing the love dare, I did that first and was not a believer, I just skipped the parts that were Christian. It taught me how to let go of him, even though I myself was still a mess inside.
Reading your sitch, I had a feeling that total restoration is ahead of you, I don't know why, but I really did. But I feel like your H is your idol still, you need to let him go and find a new meaning for your life and trust that he will come back when the time is right and he has made it through this darkness in his soul.
Sorry this is so long.
Hugs, Coly, I get exactly how you feel!
I believe I will see the bounty of the Lord in the land of the living. Wait for the Lord with courage. Be stouthearted, and wait for the Lord.