What are some paradoxes in Gone Girl?

The central paradox in the novel is captured by the title: Amy is definitely "gone" during most of the book in the sense that she is absent from her home. When the author exposes her behavior with Desi and then learns of all the steps in her intricate plot to frame Nick, the reader realizes that her mind is what is really gone—Amy is a sociopath.
Plot-wise, the salient paradox is that Amy was not kidnapped or killed; she deliberately vanished. Therefore, Nick is not guilty. But then it is revealed that they are not the perfect loving couple and he is not exactly blameless in her disappearance: part of the reason she set him up is that she is angry because he is an adulterer.