Many perps go back to the scene of the crime. On Sunday searchers/volunteers were being turned away. If he did get in, he would have to sign in (sure, he could have used a false name) -- but I guarantee you unless he was a Ninja, could he have passed LE and the search command post without being seen. Especially if he was in the water (most in the water had those waterproof suits on anyway -- at least that's what I saw on the tele).
Is it possible that he got in - maybe -- but unlikely. He'd have to be absolutely batty to think he could get to her body after 6PM on Thursday, when LE and searchers came out in droves. More stupid to be seen where the crime was committed.
IMHO Chelsea had an angel looking over her and made sure this was caught. The parents quick action certainly helped as well. He had little time, and I think the shallow grave was more like debris piled atop of her (IMHO).
There is no happy ending here. Only that Westerfield and Peterson will soon have a new roommate at San Quentin State Prison where California puts the worst of the worst.
Thinking of the family tonight --
Mel