This is just my opinion, but there are so many reasons Arron would say Crystal is innocent and she is not. So, so many, but these are the few that come to my immediate mind:
1. He claims he is innocent. If he does not say she is innocent, then that makes him not innocent, too. She can't be guilty and he is innocent.
2. She knows more info on him. He probably thinks that to show her his "loyalty" might keep her from spilling more info on him.
3. He would like to keep the spousal privilege. Saying she had nothing to do with it pretty much came together with calling her "my wife" and not wanting a divorce.
4. He wants their defenses to mesh. She is included in his plan to claim not guilty somewhere.
5. He likes to manipulate the system! Arron clearly gains some satisfaction by feeling like he can make the system do things that he wants. Him getting Crystal off charges she deserves will probably please him. Part of his diagnosis is "deception, as indicated by repeatedly lying, use of aliases, or conning others for personal profit or pleasure". That. He gains pleasure in conning others for his profit. (
http://en.wikipedia.org/wiki/Antisocial_personality_disorder)
6. Arron always acts for his own good. He is diagnosed with Antisocial Personality Disorder.
I have not finished reading yet, but I did see where he claimed to be with Crystal to get visitation with his child. He never claimed love, only personal gain. You can pretty much believe that most, if not all, of Arron's actions are for his own gain. Him saying she is innocent says absolutely nothing for her actual innocence. That's my opinion.