I think the dirty diaper is just meant to address the question of "was he even at the campground?" "Yes, he was, because there was a dirty diaper." Doesn't necessarily mean there was only one diaper or factor into timelines, just lends credence to the fact he was there. IMO.
Weird that the dogs didn't hone in on the dirty diaper or the tree after it was removed. I'm not a camper, so I'm guessing people hang a dirty diaper (and trash in general) in a tree to keep it from attracting predators. Is it possible the diaper did exactly that?