Thinking aloud here...
If the parents lied about breakfast, and Klein expanded the timeline to start at 8am, then something happened that morning. Did DeOrr die early that morning? If he did, then going to the store must have been part of the coverup and the parents must have hidden his body somewhere at that time. But then why go to the store at all? To give them a legitimate reason to be seen driving around the countryside, and try to establish that DeOrr was still alive when they were driving around, so they couldn't have been disposing of a body.
Then they go back to the campsite... Presumably ggp and IR don't know what happened, so they head off fishing with ir, and trick IR and ggp into believing DeOrr is still around. Then they "discover" he's missing, call 911 (and Vernal hauls off - why?).
Assuming the cadaver dogs didn't hit on the truck - why not?