Hi, good posts! And lots to think about. I agree with MHL that 'thank you' isn't quite right. 'I appreciate what you wrote' is probably better, as it doesn't categorize what your xh wrote.
Your response is good if he were not in MLC. As it is, it gives too much information about you. At this point he isn't really interested in you, as a person [or OW for that matter] I have come to see that it truly is a crisis and it is about survival. They aren't in good shape and they are hanging in there.
Is it an apology? Of sorts. He cannot at this stage admit that the affair with OW was a bad idea. He is at the stage of seeing how he went about it wasn't great. The point at which they can see that the affair [not even the outcome of the affair, which may go wrong] but the affair itself, was a bad idea, their world comes crashing down around their ears. because at that point they have to really face what they have done, and what they have damaged and probably lost.
I also agree that they feel second class. My xh is at the stage of giving me bits of information and sending articles he has written, and so on, a bit like a cat bringing you a dead mouse [or even live one, shudder]. He still cannot emotionally connect with me - I think this is truly the final stage.
This is what I would call an early stage apology. He wants you, imo to go on talking to him, being there for him, and showing you are OK. And up to a point, this is OK. GAG also has it right, we should not become a fixture. Reliable in that we are consistent and non threatening, but not always available on demand.
Respond, as you think fit, briefly as all agree, and then leave it. Wait to see what he does next. It is his journey. The way I feel is that i want my xh to 'recover' and mend, because I loved him and a bit of me still does. We aren't doing this for ourselves, so much as helping them along. Not fixing them or enabling them but genuinely holding out a helping hand along the way, the way you would to any dear friend who has gotten themselves into a terrible mess. You are not judging him, or luring him back, but being a friend in need.
Like I said, you rock girl. The Antonia who was could not have handled this, let alone the way you are doing.