One... omfg, I am continually blown away by how similar our Hs are on all of this. Much of what your H has said to you about the reasons why he strayed sounds like word-for-word things my H has said to me. None of it is him taking responsibility for his own actions. All just justifications to let himself off the hook for the affair. I hope you can recognize that and aren't trying to internalize all of this. I know it really helped me when I realized that my H was just following the exact same script as all these other WAHs.
Also, I'm sorry. But four children with your H gallivanting all over the world, running a business, running a home, all by yourself, dealing with major life stressors and births and depression-- I think all of your reactions are totally normal and human. So I hope you are able to take a step back on all of this and realize that yes, maybe you could have done things differently, but you were doing the best you could with the tools you had at the time, and it doesn't sound like he was being all that great of a partner to you during that time either.
On the EA vs the PA. I have to say, if there has been any opportunity for this to be a PA, if I were you I'd assume that was the case. I'm not trying to mindread here, but I do think that having crossed that line might be part of the reason that he is clinging so desperately to the justifications for why you were so terrible and why he was leaving (I notice that is what he's saying, not that he's leaving because of OW). I also wonder if having that lie out there and unaddressed, and his inability to tell you, is part of the reason he got scared off of Ring. That may or may not be helpful or true, but it just really sticks with me that there are so many other similarities and my H also swore up and down it was just an EA. I know that for him having crossed that line is part of what has made him feel like he needed to commit to the A and that being "true love" because otherwise he has to look at himself as a philanderer, not just someone who fell in love accidentally after his wife demonstrated she didn't love him with the SSM.
My H stuck to this line for so, so, so long. When he finally admitted this *was* about her and not about us, I think that was a big shift in our conversations. He really really wanted to have this narrative that he fell out of love with me because I broke him with the SSM and we were separating because it would never work out between us, and the A was a side note and not the reason. I think that was also why he hid the full depth and breadth of the A from me for so long as well-- really hurt his narrative to himself that he wasn't such a bad guy. And he is now very scared of having to face his own behavior and why he did the things he did if he recommits to the M, because in his head he doesn't really need to do that work if he rides off into the sunset with AP.
Regarding whether or not to tell the OW's H... you might look at BluWave's thread as I think she became friends with the OW's H during the time, and there may be some lessons in there for you. I agree you don't owe him anything-- you did not do this, this is 100% on your H and his W, their bad behavior, not yours. So you shouldn't internalize that wrongdoing and somehow feel you're behaving inappropriately by not saying something to the H. I would guess it might have a negative effect on the possibility of Ring with your H, so I would be really really certain that you aren't doing it partially to get the OW in trouble or possibly get her to retreat, because my guess is that would backfire. Do you know him personally? I seem to recall that she is involved in some way with your business, right? Is your H saying he is leaving in order to pursue a R with her, or just that he can't stay M to you?
Here's a thought for you to take or leave-- what do you think about asking your H if the OW's H is aware? And saying you're thinking about getting in touch with him? Exactly what you just said to us here-- that he's admitted he'd have worked on the M in the absence of OW, and you're really sad that another family is breaking up, and you really wish someone had told you if they'd known, and maybe they will have a chance to save their family if it isn't too late. He will probably freak out and get really mad, none of your beeswax, etc. But if you do end up telling him you didn't do it behind your H's back and if you stand by it being the right thing to do, I would not want to have it be a secret. Maybe that is really bad advice and I'm not recommending it by any means, but just a thought to chew on.
Hang in there Sage. You're amazing and you don't deserve this $hit.
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing