The truth always finds it way out is what I have seen here. While it is admirable that you don't want to ruin her reputation or her AP's family, is it REALLY you that ruined it or did her/their choices ruin it? Food for thought. I'm not saying you out her to anyone, she has to deal with the consequences, not you. Will it affect you, yes, and that is what you're fighting.
I've lived in the south, there can be some revenge and stuff going on in those small towns when this kind of stuff happens. I only spent about 6 years down there, so definitely not as long as you. I understand that dynamic a bit though and your concern on that front.
Sounds like you and your family are religious?
Of course she avoids conversations about ending it. Shes having a blast and cant quit the drug. Heroine addicts don't want to talk about quitting either. Until something happens to change their mind they will keep using. That something is rarely a person convincing them. Good that you've stated youre staying put in the house. She is mad that you've blown up her affair to her family? Oh well. Let her enjoy the spare bedroom.
Me40; W38; S12; D9 BD11/19/2018 D filed 12/20/18 D Final 7/2020 Being the best example I know how for my kids to see.