Well, we'll have to see if Pattie can help clear this up. I think it's interesting that (according to Pattie) Ringo didn't think anything odd about Maureen not coming home until Pattie pointed it out to him. From her account, I think Mo was trying to get R's attention. Since P and G had already drifted apart, she picked G!
I really don't know. I wasn't part of the "free love" era. But I think in both cases there was distance between the husbands and wives, and instead of trying to work the problems out (as we do routinely today through counseling), the 60s people had no such resources, so just resorted to the old "Hah! He's ignoring me... this will make him jealous!"
G has said many times about Eric, "Pattie and I were basically done with each other when she took up with him", so it was more like Pattie moved on to another relationship. I think P&G wished each other well, but weren't in love anymore. As far as G making peace with R, I don't know that much about it. R did decide to leave Mo, which really hurt her. (She tried to commit suicide by driving into a wall.) So I would have to say R was through with M also and ready to move on. But there was a good bit of wife-swapping going on in the swinging set anyway at that time (and still is, for all I know), so I don't think all this sleeping around was as shocking to this group of people as it might be to those of us from a more traditional "one relationship at a time" background.
But I really don't know the answers. I hope P's book will shed some light. At the very least, we'll learn the way she rationalized it to herself.