IMO they showered a dead or dying young man. With a basic knowledge of DNA, they tried to get others' blood off him. I can see a Lord of the Flies scenario, especially if he'd been egging people on to hit him. And a crew of drunk (and likely rolling) kids might take it too far before realizing (or caring). An actual Sharpie would take more than a quick shower, so perhaps they used turpentine or acetate, etc. and then washed him to get that off. But why bother, unless someone wrote something specific on him, and they wanted it gone. Clearly someone/s had the spur of the moment idea, likely in shock, panic and still drugged, to make it look like he was hit by a vehicle. But any kid who grew up in the age of CSI type shows knows involved=accessory, so I understand the silence. Most of them have an idea what happened, and possibly only one, two or a few actually dumped him. I believe out of that many people (over 4 days coming and going), someone will crack, or plea. What's interesting is if he was initially attacked with hands, at least one person had his/her own damage in the days following. It sounds like it segued into planks or stoning or who knows what, but the majority of injury was done w/o hands. The coroner can tell at least the type of object that strikes someone, whether or not it's been released or not. I'd imagine the family has info they are likely not sharing.
I'd guess he was dragged at a high speed, torso down, perhaps off the back of an ATV. Hopefully already unconscious. There'd be more evidence on the road if he had been dragged there, even if he was already dead. Any samples embedded in him would be tested, so they likely know dirt, sand, flora or macadam and are waiting for someone to talk. What is giving me pause is his teeth across the road. Either he was dumped and when his mouth hit they scattered, or someone hit him one last time and they came out, but the scene sounds too clean for that. Maybe someone collected them at the time they were broken (getting them away from the house/property) and cast them across the road. That's forethinking. Which makes the whole thing even more horrifying.