I tend to think there is no point in pushing him to do more discovery. Just as there is no "why" that could really explain it to you, there is probably also no "why" that could explain it to himself either. It might be sufficient for him to have concluded that he was an absolute d1ckhead, a complete fool, and an embarrassment to himself.
I think about people who fall for scams, like sending money to some guy in Nigeria who has inherited a billion dollars. People actually do this and they aren't even necessarily stupid. You can ask them why they fell for it, and all they can really say is that they were idiots, and they can't understand why they were so foolish. The difference with an affair is that that it's more self-inflicted, but it is still (in some types of case) just as much a case of "why on earth was I such a twit?" Ask a student, "why did you spend all week goofing off and playing video games when you knew you had an exam coming?" There's no answer, but they can still learn a lessong from the damage done. So - there really could just be no actual answer to these questions, or an answer so stupid that it doesn't bear analysis. (There are other kinds of cheater, who know why they did it and who are going to keep doing it, who are essentially always living falsely, but that doesn't sound like what you have.) This is contingent of course on his having left the craziness behind. You can only go with your gut on that. If he is a borderline personality who can easily deceive you then who knows? But it doesn't sound like he's that type. I get what you mean about feeling like you are the consolation prize. Although my wife confessed her affair and "chose" me over her AP, this helps less than you think. We have small children and I earn three times what my wife earns and (literally) ten times what her AP earns. She is embarrassed by the affair and doesn't want people to know. So there's a part of me that thinks she is just faking it for the kids, for the money, and so that I don't tell anyone. My point is just that in every case of cheating the BS feels like the consolation prize; it's inevitable. As for changing from someone who doesn't enjoy your company to someone who does - I get that too. It's a pretty consistent story here that cheating spouses treat the BS terribly. My wife was really revolting. There is nothing okay about it, but it seems like it is a natural consequence of the psychologically weird place they put themselves into. They are experiencing a combination of smug superiority (because they have their cool secret) and a need for self-justification. For whatever reason they are not in a place where they can be self-critical - if they could be self-critical, they wouldn't be cheating. So the construct, consciously or unconsciously, a false world-view where (a) they have something special happening that makes normal rules inapplicable and (b) you wouldn't care if you did find out and (c) you are nothing more than an irritant in the way of their "real" life. There's also this element of treating you as a kind of baseline item. Like a kid who knows that no matter what they do, they'll still have their mother - cheaters think that no matter what they do, they still have their marriage. So that kid treats his mother like cr@p, and cheaters treat their spouses like cr@p. Like I said, none of this is okay. However it defies explanation when they come out of that mental state so they may just genuinely have no answer to the "why". The "change" from non-cheater to cheater and back to non-cheater is not normal human development, it is some kind of switch between states, like delirium. Just one more thing, the not remembering. I find this bizarre. However, I am absolutely convinced that my wife does not remember a lot of stuff. Whatever is going on during an affair and in the aftermath (some of these things were in the post d-day crazy time), I believe it can cause the participant not to remember. My wife was *insane* during and after her affair. I am not saying that to excuse her (I do not think it is an excuse at all) but simply to report an observation that I am confident about. She was nuts. She was making decisions that were illogical, against her values, and against even her own selfish interests. She said stuff she can't remember saying. I have rambled a bit again, sorry 😉
Maybe it is okay, maybe it will be okay.
BS, d-day Feb 2017, 16 mth affair.