LG - I just don't know that I see much useful information in this last communication. All their amateur psychoanalysis of your H seems a little suspect to me- especially since her fiance seems so weirdly invested in his role as white knight and rescuer, and BMOW seems pretty happy to blame all her problems on your H now that he has rejected her (I mean honestly, he met her on a swingers website and he WASN'T the first guy she met there!).
Let's try to ferret out any useful pieces, okay? - this issue of H being "molested" at 14. Did he ever tell you about it? And it was by a 16 year old female babysitter? While yes, there are some situations in which this could be abuse - it seems more likely that it was sexual experimentation by two teens close in age. After all, what kind of threat or coercion could a 16 yr old girl have used against a 14 yr old boy?
- H feeling inadequate about his role as provider - okay, probably a factor in many mid-life affairs - feelings of inadequacy leading to finding a "lesser" person who would look up to them. But frankly, this might explain AN affair but probably not the multiple casual encounters described
- a fetish for BBW - yes, some people take advantage of them because they are easy prey - others have a genuine fetish for them. I suppose if the 16 yr old babysitter was fat, that could have been the start of a fetish, since early sexual encounters leave a strong imprint. If so, though, I'm surprised he wasn't trying to "fatten you up" during the marriage. This could all just be a line he sold BMOW so he could keep sleeping with her. Who knows?
- To me, all that seems clear or useful out of these communications is A) that he met her and others on the website - you already knew that is where they met, so this is no big surprise. B) that he told her he'd been sleeping with prostitutes etc. as far back as the 90s - possibly true, since I'm not sure what he would have gained by telling her that. IF he told her that.
And these two pieces of info tell you that your H has some kind of problem (maybe sexual addiction, maybe he's just a sociopath, maybe he's just a jerk) - and that his problem puts you at serious risk for STDs.
Now it's up to you to decide what it would take for you to be comfortable in this relationship (complete transparency? H confesses everything and gets intensive counseling for at least a year, with you welcome in occasional counseling sessions? Lets you install a keystroke logger on his computer so you can monitor?).
Or decide if none of that would ever bring you to a point where you could be comfortable again.
Either way, I'm not sure BMOW and her weird fiance should play any more role in your life. They creep me out.