I could see them taking Dehner's body back, but since it was a huge-ass rock formation that fell on Gary, I'd say his body was probably pulped. Best to just leave it there than bring back a few scrapings and some teeth.
I would have left Mitchell's pulped body behind and gotten out of there fast. I'm reminded of a line, I think it's from one of Anne Rice's vampire novels. A very old, weary vampire is desperate to die, so he arranges to have someone burn him. And he says, "Scatter the ashes, or I might rise again. And in what form I dare not contemplate."