What I think is the ticket is clear, honest demonstration that she can handle a new relationship at a POLY, PARTNERED pace instead of her SINGLE pace, which she admitted was the deal. She hadn't dated anyone on her own since meeting me two years ago, and picked up where she left off in her high-speed dating habits, instead of checking herself. BAH.
IF this is the agreed upon goal for AFTER the repairs have been done on the individuals and the dynamic duo?
I would entertain a trial "V" polyship of her dating her person along with you for a month. And still keep you guys apart from each other. You can observe if she gets all NRE woo woo and manages her ORE rights and responsibilities like a Muppet or like a Jedi.
If Muppet still? Ciao! Game over.
If Jedi? Alright. Then you can take the next baby step in thinking about repair work with your meta person so you can Change there too -- from the cheater partner you feel GRR about to a polite, cordial relationship of acquaintances who share a Sweetie.
But they cannot be RUSHING you into best buddy metamour land! That is a nice want, it is not a NEED, and you have serious limits to overcome that got foisted on you by RUSHING in the first place.
Sometimes asking for too much to fast is asking to be allowed to just be plain ol' FRESH
and taking liberties!