Firsts are hard to get over... I get that. HOWEVER, have you even been talking to him recently or have you been planning all of this all on your own? Because from what you've told us, there is no indication that either one of them is interested in a relationship with you.
If by some amazing piece of luck his fiance would be interested in poly and you are considered, you would need to be upfront about EVERYTHING. How you and he originally met, how he cheated on her with you (and do you realize he lied to you as well? Have you dealt with/thought about that?), how you at one point wanted to sabotage their relationship so that you could have him... All of that would need to be addressed so that you could all start from an open, honest place. If after hearing all that garbage, she was still willing to try it with you... Well, I would be amazed.
You also have some things that are contradictory in your post - you say you want a closed triad but you'd be okay with her dating. That would be more open, don't you think? You say that some other person started all these rumors about you that made the fiance dislike you but then you say that at one point you DID want to break them up - sounds like grounds for disliking someone to me.
This situation is so messed up with so many screwed up parts inside it (fiance being lied to and cheated on, the guy lying to both of you, you possibly borderline stalking, no clear or honest communication going on... Shall I continue?). I think it's best to seek some therapy for yourself, get over this guy, and move the heck on. Sounds like the other two could use some counseling or at least some work on communication too if their relationship is going to last.