This is similar to a theory I have, only I put the mysterious GGPA (we STILL haven't heard from him) at the wheel repositioning the vehicle - to me he's the 1 person all 3 would lie for.
No disrespect for your opinion but I think GGPA would be one of the least people JM or VDK would lie for. I could see IR covering for his friend but I couldn't imagine any mother covering for her grandfather. The only person I could see her covering for is Vernal, and mostly one because she fears him or feels she cant live without him (financially and or emotionally). So what if GGPA loses his licence. He is older and being cared for by family anyway. Unless there were a lot of drinking and/or drugs involved on the part of all 4 adults, I would assume that Deorr being hit by the truck during the set up of camp at dusk would be nothing more than a horrible accident. Nobody would go to jail for an accident. Calling 911 as soon as he was noticed would be the logical thing to do.
When we were camping in Idaho last summer we nearly ran over our dog (leashed to the bumper of our truck) when backing up to hook up to the trailer. We were all sober. The dog had crawled under the truck for shade. My husband then got in and started backing up, not able to see the dog or leash in the rearview. Luckily I screamed for him to stop and no harm was done. But based on this experience I could totally see how a small child could be in the vicinity and not seen by a driver. A toddler's head would be about bumper height. If there was any significant blood or tissue on the bumper I would have assumed it would have been noticed in the initial search and media would not have gone on an on about a child either getting lost, abducted, or eaten by a wild animal. It should have been obvious from the beginning that it was an accident/cover up, unless the truck was cleaned well (maybe the real reason to go back to town Friday morning?) and no evidence was visible to the naked eye.