I’m sorry, I didn’t get the first part of your message, but I guess this one is enough to understand your point, which I’m going to disagree with.
A nonexclusive relationship between them has never worked in my book. I think it could have been a thing back then, when they kinda both were a mess and young and full of resentment and frustration they may have fucked out. But I believe in the last decade, things have settled down, they’ve matured, and considering their history, the deep feelings they have for each other and the mutual love and respect, and I don’t think it could still be a thing. One always ends up hurt in that kind of relationship, so I think it would show. Plus, they have kids who are old enough to understand these things, and because they’re friends and also co-workers, it would be impossible for them to hide such a relationship from their children for so long. Finally, a nonexclusive relationship would mean that their respective partners would also have to bare with this agreement. I’m not going to talk about their previous partners/spouses, but PM doesn’t look like the kind of man who is ready to share.