Laurat859I've not read all posts but I have read all of your posts. I am so sorry this has happened, it is terrible.
I'm with alarox "The sort of future you imagined with your DP and his family has been shot down in flames. I feel for your DP but in your position I'd get the hell away from them all." He could have told you this a long time ago but he foolishly waited, I say foolishly because I think waiting made it much worse.
Even your opening post showed your potential MIL to be very manipulative. She is expecting overnight stays! "She said she needed to know my feelings because when she has grandchildren she wants to see lots of them and be able to have them stay over at their house etc etc."
I adore my parents in law but my kids don't stay over with them, there is no risk, there is just no need to. It was never needed when they were little and both my kids are real home bodies. I never understand grandparents who automatically expect so much from new parents, to give up their kids overnight etc.
Also, "She trusts him completely that nothing will happen again." That is not something she can promise. She may trust him but she can't make you trust him as she cannot for a second guarantee anything, and neither can he.
PinkPanther27 Sun 22-Oct-17 00:14:09 makes some very important points.
Her judgement is totally out to allow this man into the house with her teenage son, that means she will not make a good grandma, that ship sailed the day she knowingly allowed a paedophile around her own child.
ElizabethDarcey is right "... the right reaction and decision, the one you can't quite see through the fog is - I WILL NEVER LET A PAEDOPHILE ANYWHERE NEAR ANY CHILD OF MINE, EVER."