Maybe it wasn't an adult alligator snapping turtle that attacked the bodies.
I found this:
"hatchlings and juveniles usually live in small streams"
at http://www.arkive.org/alligator-snapping-turtle/macrochelys-temminckii/
Sorry, but "the people claiming animal predation" are certified forensic pathologists who have years of experience examining bodies and photos of animal predation. These experts have not definitively said that the animal predation was alligator snapping turtles or any specific animal. One expert (Sptiz) speculated that the attacks were by canines (dogs or coyotes or wolves, maybe). The fact is that the experts have only testified that the wounds were animal predation, not knife wounds. Beyond that is nothing but speculation, but the experts were clear that no knife was involved.
The bodies were underwater, so the range of likely predators is very limited. It would have to be animals that swim/live in the water. Again, given the branch was very narrow and shallow (too shallow for fish other than a possible minnow or tadpole) and with with steep, solid banks and few areas for snapping turtles to burrow, live or even swim, it seems highly unlikely. Hatchlings or juveniles would also be unlikely, since, again, the area around the branch/crime scene is not conducive to building snapping turtle nests. They need large areas of mud, gravel or sand for that.
Add to that the fact the bodies weren't there very long and hadn't much time to begin decomp, freshwater animal predation seems highly unlikely IMO.
That said, I've never made up my mind about who the murderer(s) were. I've just always felt the snapping turtle theory was a red herring or simply an honest mistake. The experts may be forensic pathologists, but they're likely not experts about wildlife habitats in that area.