I don't disagree that EA should be looked at carefully, but I don't think these new revelations are proof of guilt either.
Polygraph: I don't blame him for not taking it. He looks like a deer in headlights just being interviewed for the news. I would think he would be at a high risk of failing regardless.
Erasing cell phone history: Could easily be trying to hide involvement or knowledge of other activities that he didn't want the police to know about. Maybe drugs? I'm really surprised that police would tell Christina's mom this as well.
Not helping with searches: To me, this could prove his innocence just as much as guilt. If he did kill her, helping with the searches would be a diversion.
Car search/seatbelt missing: Even if CM left with EA, having her seatbelt on would indicate to me that she left voluntarily.
Denying parking in the garage/walking Christina to her car: This was stupid. I lean toward him knowing something, but being put in a position where he feels like he can't tell the truth. Maybe he knows who she left with, where she went, or what her plans were?? Maybe she mentioned something during the walk to the parking garage that he doesn't want to tell? On one of the interviews with the news stations (can't remember which one), there were camera shots of texts that he had been sent. One of them mentioned telling them where Christina was so they could go pick her up. They (the sender of the text) didn't seem to imply she was dead, just hiding somewhere, and that he knew where.
Just my thoughts at this point.