I'm wondering why you're so passive in your own life and relationship, Pomo81.
He lied to you repeatedly, and the only reason he's not currently continuing an affair with this woman is not out of loyalty, love or respect for you, but because she is - at least for now - rebuffing him.
He still wants to, though.
He is obsessing over her, and monitoring every single inconsequential social media post she makes, and unconciously mentioning it out loud when he mentally files away for future reference information that he hears about the changing divorce laws (?!?!).
... As far as I can see, he takes you, and the idea of a life with you, completely and utterly for granted.
And you're letting him.
You don't seem to require an apology, remorse, therapy, boundary work or better treatment as a condition for him staying with you.
No. You just want to 'watch him'.
For what? What's the point?
You honestly seem almost content to just bob along and wait in the wings for him to get over being dumped after a bit and choose you by default (um... yay?).
...He's certainly content to let you - weeks out from your wedding, at the time when he should be most infatuated and in love with you, his fiance, and instead he's mooning over her.
Your posts suggest that you find an odd (to me) degree of relief in the notion that this woman has currently told him that she doesn't want to pursue an affair with him.
Your relief presumes that she is the threat to your relationship.
She's not, he is.
He is what's broken in your relationship, not the fact that another woman exists who finds him attractive.
He pursued her. He lied to you. He shows no remorse, no personal insight, no drive to address and fix whatever led him to betray you. He continues to moon over her and behave with poor boundaries.
The fact that this woman has now put up a boundary does not fix him, does not fix what is broken in your relationship.
What if she changes her mind?
What about the next woman that comes along and shows him a bit of attention?
Pomo81, I get it. You're weeks out from your wedding and you don't want to cancel/postpone. You've been together for ages and sunk cost fallacy etc. etc.
But if you're intent on staying with this man, to ever have a hope of a real, stable relationship you must draw a line in the sand.
Right now you are literally teaching him that he doesn't have to respect or honour you.
That he can betray you and take you for granted, and you will do nothing except obsess over him obsessing and cross your fingers that he 'chooses' you.