Peter, I've heard fantasies from some who didn't reach piecing that their ex would beg for it. That doesn't match my reality or most situations I've read, but when you're at that stage their desire is obvious and they're making it happen. You don't have to guess.
It sounds like she's currently in more of an uncertain stage where she's considering what she wants to do with her life but you expect that to change. May opted for Discernment Therapy at this point.
The good news, if you get to piecing, you don't need to build trust in a day. That's a common concern! It typically returns but can take months or years of consistent behavior.
You should consider what you require for piecing. No requirements are a common cause of failure here. When sneakiness is involved--I can't recall if your W told you it was over before sleeping with OM--a common requirement is access to her cell phone, e-mail accounts, etc... and not vice-versa. That would help you rebuild trust that the sneakyism is over.
