The talk you had with your husband, in which he seems to have been more honest, is clearly a step forward. However, my concern is that discussion was essentially powered by you. It feels like people here have advised that he has got to be more honest, and you have made this happen. The initiation of the talk, and the total honesty should have come from him.
I don't mean should come from him in an ideal world, I mean has to come from him if this relationship is to survive.
You seem to be the one the making the superhuman effort while pregnant & working to fix this - laying out your heart on a plate on a public forum, at a time when you are immensely vulnerable, which takes a great deal of courage, because you are so determined to save this marriage. You are clearly a fixer and that's not necessarily a negative, but your intended goals need to be achievable otherwise you are simply throwing away time and energy.
I don't get the impression your H is working as hard as you to cover the ground necessary to rebuild this relationship. And I draw your attention to it only because it is an indication that he may not have the stamina for the long haul that trying to fix it. Because it's going to take a hell of a long time.
It's interesting that he has agreed to "take steps to deal with his anger and frustration - ramping up his physical activity a notch". He comes across in your account of him as an angry person. It's good that he has acknowledged this as I think his anger may be a problem in itself, quite separate from the infidelity issue. I think it's naïve to believe that physical exercise will do much - it can help to burn off the effects of the anger - but to really address it he needs to look at its roots within himself. That is another long project.
This sentence in your latest posts struck me:
What I am saying is that I don't think he would have strayed with any other woman, because no other woman would have put the pressure on him -and on me- in precisely the right sort of way, she has been the perfect storm of cuckold.
I think here you are slipping back into self-deception here. What you're saying is that if only the cards had fallen differently he wouldn't have been tempted, which puts all the onus on chance and not on his own weakness.
In fact, the obvious inference is that if he could cheat with his wife's best friend in the early days of marriage, when she is a guest in your house & apparently vulnerable, then he could cheat with anyone. Who is to say another woman won't put 'pressure' on him in some other situation? If his fidelity relies on no other woman ever being in a position to make advances & pressure him, then you have no security.
One further thing is that you have not posited any reason why he chose to be unfaithful. I don't think it's just about weakness & acting on impulse, opportunity. I think it may be a message, an angry or even passive aggressive act. A message with his will and his body that he is perhaps not able to speak in words.
What it communicates to me is that on some level he that he may not be comfortable in this marriage. Not necessarily consciously. Not necessarily that he doesn't love you, nor that he doesn't want to have children. But that for some reason, he is struggling with the relationship and/or himself and that was the way he manifested some inner issues. If you say that he did not cheat on previous gfs, although of course this is difficult to be sure of, then the implication is that there is something specific in his relationship with you that is problematic for him.
I am going to speculate here, and as I've never met him it may be completely unfair, & I acknowledge that. You come across as intelligent and articulate, perhaps more so than your H. I wonder if you are too intelligent for him, that he feels that your standards & expectations are too high, that he cannot live up to them. It may be that you were someone he aspired to marry but finds that he does not feel comfortable married to. I say this as no criticism of you, I don't mean that you are doing anything wrong, I'm just questioning whether you are as compatible as would like to believe. Anyway its just a suggestion.
Because even taking temptation, weakness, & a particular concatenation of circumstances into account, if he had been absolutely 100% committed to this marriage, he wouldn't have jeopardised it.