One, you're not in piecing if he brings up the possibility of R. You're in piecing when he is ready to COMMIT to doing 100% to rebuild- AND he is willing to back that up with committal action (letter of no contact, passwords on phones/email, etc.). So it might be a good idea to know what you'd need to be black and white about whether he met YOUR requirements for piecing. If you're too desperate to have requirements you will ENABLE his As and be responsible for the failure of your M. You must be strong!
Two, at that point the details of the affair may not be something you want to focus on. Listen...and we're in fairy land now, but if that comes to pass there is NOTHING he's going to say that will restore your trust or take away the pain. Something was destroyed that will never completely heal. It stinks. So digging digging digging to try to get answers, clarity, etc, that may drive you insane and destroy the chances of R. Yes, you have a right to have him be honest to a point if it helps with closure, but you also will have to be able to let it go.
Letting it go will be as hard as letting him go now. In a way this is practice for acceptance.
But right now talking about the A is futile. Yeah, he lied. Don't tell me that "It's one thing to have an A, but to LIE to me..." If he lies again and you know it tell him to stop, that you know he's lying, and you won't be disrespected that way. But other than actively shutting down a conversation in which he's lying I don't see any reason to focus on the A at all. It it pursuing, controlling, and not effective.
PS- be warned. Rebound relationships can come and go. My STBX told me a few times she broke it off with OM. One time she told me that at 2AM and was crying that she missed our M and was thinking of ending it all. I went over there because I was concerned about her...only to find OM sleeping in the bed. So there idea of 'breaking up' is sometimes nothing more than intent. Or a mood.
DETACH. GAL. LONG JOURNEY. PLEASE!!!
Me:38 XW:38 T:11 years M:8 years Kids: S14, D11, D7 BD/Move out day: 6/17/14, D final Dec 15