I am glad things improved OP and I hope it all works out.
Most of the advice on MN is based on the standard 'script' that cheaters follow, with good reason. Time and again it's the same. Hopefully your case is different, there do have to be variations.
Your script as I read it: The unmarried man in a relationship gets involved in an intense friendship that turns romantic. He keeps his options open but essentially sticks with his original partner. Doesn't have sex with the woman although other boundaries are overstepped, emotionally as well as physical. This frustrates the OW and she builds the relationship up in her mind and pursues the man. Meanwhile OP bides her time, unwilling to leave the man she loves. He doesn't ditch OW outright, perhaps preferring to avoid the conflict, particularly if they work together. Perhaps still keeping her on the back burner but still 'choosing' OP and deciding to make a go of it with further commitment. He's been honest with OP about the contact he has had with OW, maybe less honest about all of his motivations but still sounds like you're out if the other side of it for now.
You know as well as we all do here that there's no guarantees that he won't cheat on you in future and you will have to be vigilant. I'd also ensure that you're legally protected and do some divorce planning in advance of marriage. Prenup, separate bank accounts, hidden slush fund should you need to split, protect your own assets as best you can ... and hope!
I get that loads of people will pile in and say you're doing the wrong thing but it sounds like a reasonably happy resolution for you. It might not be a 'forever' solution.
Could you tell him you'd feel so much happier if he made it clear to OW that he didn't want to hear from her again and block her as best he could?
Good luck!