Lisa, I'm very impressed. Your last post should be 'stickied' as a way to handle the boomerang spouse.
One thing to consider, and I'm not suggesting you need to do this immediately, only to start thinking about how and when you'll work this in...applaud what you do see.
I'm not suggesting that you leap back into his arms, or that you close your eyes and trust him completely, or anything like that. But I think it IS ok to give him some positives when you can. He is battling an addiction, and you want to assure him that the life he can have with you will be ok. Men need affirmation from their W's, and he needs it now more than ever. You may feel he deserves it less than ever, but that's debatable. While it's true he has enough lack of character to betray you, he also may have enough to avoid a trap that would end many marriages, and someday you might actually respect how quickly he aborted his MLC (assuming this keeps up!).
These are just brainstorms, but maybe some reassuring words could be:
-While I am taking no responsibility for your behavior, I will say this time has allowed me to reflect on my contribution to the breakdown of our M. If you are able to demonstrate your recommitment to the M it will take hard work, but I want you to know that I'm willing to put in that hard work as well, so that we can build this M into something better than it was before. It's important for me that your needs are met so our M is safe, and I'm willing to take a look at anything and everything that lead to the spot we ended up.
-Given the chance I would much prefer to remain M to you, and rebuild our M so it was better than before and we both felt more understood, appreciated, and fulfilled. You're my H and of course I love you. Of course, this doesn't change anything I've said as you know my boundaries, and that while it would hurt to be D that would be the only alternative if you don't respect them. But I do want you to know that I'm willing to do my part, that your needs are important too, and that I would be satisfied with nothing less than an improved M that we were both happy with. It's a tough road, but it's real, and maybe we can even have some of that fantasy happiness in our own M. I can't lower my standards, but I am in your corner.
I dunno. Tweak as needed. But I do think you have to give him some hope and encouragement without lowering the bar or thinking he's getting 'free passes'.
Does that make any sense?
Me:38 XW:38 T:11 years M:8 years Kids: S14, D11, D7 BD/Move out day: 6/17/14, D final Dec 15