I wrote those questions when I was only half way through the book! Having finished the book I think I might have asked different questions.
At first I thought Nick was your average slightly selfish creature - marriage has gone off the boil, lets look elsewhere for a bit of excitement - until I got further into the book when yes, I agree with you, they deserved each other!
Wonder how no one had ever really seen Amy for the devious creature she was - in fact more than devious, a sociopath.
I thought the parents were quite ghastly! Although the plot had many twists and turns, some quite clever, it wasn't very believable and as a book I don't think it was particularly good or memorable.
As you say, I prefer my own honest relationship!