I'm not sure if my advice/observations will be helpful, but I wanted to interact to let you know I remain interested and committed to what you're going through.
First, before you ever referenced it, your post made me think of your previous post about feeling numb. I remember writing something to you about how I thought I was in the same place and then found myself having a 5am nervous breakdown a week ago.
I'm actually glad to hear you had another "rupture" of some kind and gave yourself permission to just let loose with your grief and pain. Get it out. You don't have to be strong 24/7. Letting yourself feel the pain and cry literally releases those chemicals from your body and you can't help but feel a little stronger/better/whathaveyou afterwards on some level. And you don't want to bottle things up because when there is a break/trigger, you don't want to be caught off guard by your reaction.
Second, this does sound like a version of cake eating to me... but maybe not a typical one. What he has done to you is SOOOOO awful that I can't imagine that he actually wants R or believes it's truly possible. But I picture myself in his shoes, leaving you and his baby for the night, looking back at the two of you as he hits the door, and he just happens to be human enough to recognize what a beautiful sight that must be and feels love. Who wouldn't? It's like he has you in a little box or snowglobe that he can look at when he wants to because he doesn't yet have to face D paperwork or you with a new partner or the rest.
And then he gets to go off and still live his crazy double life. It's like having the best of both worlds.
If it were me and my WS said that to me and our (four-legged) DD, I wouldn't be able to help myself but say something like "THEN ACT LIKE IT" or "you have a great way of showing it" or "did you just realize that" or... (you get the idea)
Frankly, if you want to be less of an asshole than I tend to be when I'm pissed off and hurt, I do think it's reasonable to say to him "you know what, it really hurts me when you say that because I just can't believe that it can be true given what you've done, so if you really mean it, I hope you can respect that I'd rather not hear that right now"
But, again, I preface all of this with the fact that I am by no means a veteran!