SG is there some particular reason why this conversation has to happen tonight?
If you aren't prepared, and I don't think it sounds as if you are to have this chit chat then I would leave it for three days or so and tell her you need a few more days to prepare...
You do NOT have to accept things on HER terms simply because she is holding the knife of divorce over your head...
This is a poker game SG and right now she doesnt' have very good cards from the sounds of it...
She's got many years with you, her mother isn't supporting her divorce... SHe's cheating on you and you are prepared to expose it publically...
Once you expose SG her credibility for pursuing divorce is SHOT to HELL.. She will NOT want to go that route after you have painted OM out to be the stalking creep he is..
You do NOT have to cower in fear here and just meet on HER terms and talk about what SHE wants to talk about... Do NOT let her lead this marriage into a ditch
You talk when you are BOTH ready to talk You meet where you are BOTH comfortable meeting YOu talk about the TRUTH and don't let her side track you with hollow negative naysaying BULLSHITE