So IF Deorr was alive and well at 1 pm when he was left with GGP while the parents and IR went for a walk, how long were the parents with IR? Maybe Deorr decided to catch up, maybe GGP even told them he was headed that way. Maybe they were a little irritated that Deorr didn't lay down and take a nap so they could do whatever they were wanting to do. Maybe she did keep looking back to watch Deorr walk up to them. By that time, IR may have been far enough away to not hear or see what was going on. Maybe they tried to send Deorr back to GGP a few times before getting mad and lashing out. IR's busy fishing, GGP sees Deorr walk to his parents, so he retires to the camper.
Let's say that only took 10-20 minutes. So we're sometime between 1:10 and 1:50 pm give or take, leaving 35-75 minutes to hide a body from IR and GGP. Because until they call 911, there's really no one else to hide from. In a rush, they are forced to make a quick decision, call 911 and risk going to jail or come up with a story. I imagine it wouldn't take long in a panic to make the wrong decision. They decide to go with lost child in the wilderness and realize GGP is an easy target. Jessica gives Vernal time (probably 10-17 minutes) to get Deorr out of viewing distance and back to Jessica before they play the "where's Deorr?" card. GGP is too frail to hike up a mountain to search, so he starts looking around camp. Maybe they direct IR to look around where he was fishing in case Deorr tried following him. They take off towards wherever Deorr is or maybe even split up. Maybe they tried to dig a hole, maybe they tried to bury him in a creek, whatever they tried didn't work and after 20-50 minutes or so of trying, they realize they need a better plan. Vernal decides to take him away from the camp and they agree on dual 911 calls after so many minutes. Vernal would have bought quite a bit of time to drive him a good distance away and if SAR/LE showed up before he got back, JM could have just said he drove up on one of the roads in case Deorr was walking them.
That's the scenario I'm sticking with until some new info comes out.