Hey my dear projecting is super normal. My bff's H would get just absolute crazy right about the time he would start approaching that line from friendship to EA. Now they got through one major PA and one major EA but they had been together since they were so, so young. I think learning to navigate that stuff in your early 20s is hard. I'd have to say there were exes and other female friends of friends that he might have gotten a little too chatty with over the years, but I wouldn't call it an all out EA. No "I love yous" or "oh if the world was different we could be together" crap. But when ever he was behaving a little in appropriately he's automatically start projecting on her. By their mid 20s she could sniff it out. As they near 40 he's just gotten much better at protecting his W vs feeding his ego.
My exH was just literally so insecure he constantly accused me of cheating. He went through my journals, my computer files, my emails, my IMs. Anything he could get his hands on he was constantly playing detective. All it really did was teach me how to hide my affair when I finally did have one. But he also knew he treated me like crap and I could do better. A lot of it was his fear that I would leave him, and his desperation for it to be my fault. So much so when I did leave, 4 years after my actually affair. He told people we split because of my affair. Not his drug use. Not his drinking. Not his mental health issues. Not his inability to carry his share of the household or parenting. Not his constant criticism. Not his inability to make his child a priority. Definitely the affair I had 4 years prior.
Given what I know about your H, I'd say a little of column A and a little of column B is what's going on here.
