@Sazdun
"Sounds very churlish but I would almost like her to see we have managed to work it out and stay together despite her. Emphasise that she was nothing in the long term. Sounds like I want to win I know and in reality I have won a bit of a shit prize but us working on this just seems to sit or feel more right than I have felt in weeks."
I don't see how you can and that is no reason to just put up with a ridiculous manchild like him just to prove you have won the booby prize. I've read all of this thread and the previous one over the last few weeks - I have probably lost track of all your personal circumstances - but I don't see what there is to save.
All this angst about the other woman, friendship groups, saving face - I don't get. Over and over, tying yourself in knots. Your kids will get over not seeing her.
He's not just a cheat, he is a liar, he is spineless, incredibly immature and I have no idea why this circus is still going on. He's pathetic, he is not going to man up, grow up or whatever, so you have to do it.
Do you have any friends who are not part of a "friendship group" that are just yours? Nothing to do with him, or his workplace, or his other woman? If you have, start there.
And you cannot trust this man again. What a complete weasel he is.
And he will probably see you as the booby prize, too, because he can't have her. Do you really want to grow old with a pointless pillock like him? I'd be filing for divorce.