Memory Lane - I think if you re-read your posts, you might see how conflicted, contradictory and in denial you are about all this. As you are in a bad place at the moment in terms of your sense of self, it is to be expected though.
It seems highly unlikely to me that his marriage is ending - did his wife confirm this?
Regardless of their situation, he is treating her with contempt - and I see her reminding you that their 30-year marriage has lasted for a reason, as her understandable resistance to his attempts to re-write history. She probably knws only too well that he has been saying their marriage was a rebound thing, done out of duty because of pregnancy, which cannot explain why he is still there when their youngest child is 26. He is voting with his feet - and they are firmly ensconced in the marital home.
Old flames are indeed the most dangerous, because they evoke in us memories of a time when we were young, carefree and when life seemed full of possibility. Your exH is idealising to an extent and is feeding his (and your) "long-lost love" fantasy. Yes, he may have felt vaguely guilty about hurting you years ago - and he may have wondered "what if?" - and in fairness to him, this may have stirred up some uncomfortable feelings in him, probably not necessarily related to the past, but more because you represent an opportunity for him to be unfaithful.
That's where my sympathy for him ends though. Setting you a trap to see if you were a "bunny boiler"? Do you really think that kind, emotionally intelligent men do things like this? If his marriage had ended, what would it have mattered if you had been invested enough emotionally to pursue a meeting with him?
He promised his wife he wouldn't contact you again and he's broken that. He is portraying her as an angry Mum who supervises his interactions, thereby shirking responsibility for his actions. She sounds like a decent woman tbh - and I feel sorry for her.
As for why this happened, it's pretty obvious isn't it? You are at a cross-roads in your marriage, going through the menopause (from your "window" thread) and your husband has moved out of the marital bed. It's not surprising in the least that this has happened now, but you must know it's not the answer. Your ex never was the answer.
You don't say much here about what is going on in your marriage or how feelings are communicated, or indeed whether you want your marriage to continue. If you do want a better relationship with your H, it seems to me that you are delivering clues and hints to him (leaving the window open, despite his discomfort) without telling him directly how bad things really are. He in turn is leaving bigger clues for you, by exiting the marital bed rather than trying to reach a compromise about the room temperature. What seems to be missing is an honest conversation about what's really going on here.
If you want your marriage to work, I do believe in telling him what has been happening here with the exH. Given that you two seem to communicate so indirectly, it could be that something as incontrovertible as your actions with the exH might be the sort of wake-up call your H would respond to, rather than passive-aggressive attempts to express your discontent with one another.