Well, Wonder, at the very least (and it is pretty paltry, I must say) you finally have your answer to why he gave you the papers...
He has decided in his head (which one?) that he will be happy with OW... so, it was necessary in his head that he get the D... Also explains why he couldn't tell you, cause it is a stupid reason... Just more mileage in their journey.
You know, I know and he WILL know she isn't going to make him happy. Whether you stick around long enough for him to figure it out is YOUR call.
I'm sorry your MIL has chosen to betray you as well. Everything you are describing was typical for my Hs family. If one of them cheated on their spouse, it just wasn't an issue... they never seemed to make anyone own up to their mistakes. I can tell you, it wasn't pretty. Oh well. Maybe its a good thing that Hs mom doesn't know about his A... I'm sure I'd find out just how much thicker than water blood really is... More anger to process, I know...
Relax. Appreciate. Be calm. Laugh. Enjoy. Be secure. Be loving. Be loved. Don't personalize. Don't ASSume. Accept. Be grateful.