Your situation's kinda cool man for some reason it's been an hour and I'm still on the same subject.
I was thinking about it and I'm not saying my initial thoughts weren't accurate, but I'm trying to figure out a way that doesn't involve getting dragged along.
I'd be really interested to hear about what happens if you called her, told her you wanted to have her over for dinner some night and make it clear it's a one on one thing. Based on what I've heard I think you could seriously nail her here at any time if you play your cards right. I'd be even more interested if as soon as she came in your door you put a move on her and put dinner on hold for awhile. I think it'd work. She's known you for eighteen years? Sounds like the only thing she doesn't know about you is from here to the bedroom and given the fact she obviously likes you, we wouldn't wanna keep our lady in waiting, would we?
Younger girls cheat on their boyfriends because they meet someone else who looks good and makes them feel safe, all the while being less boring than their alternative. Some things never change my friend.