Thing is, "Spent 8 days wandering around by a relatively well-trafficked and inhabited trail that everyone including their own families claims it impossible to get lost from, somehow dodged all the search parties looking for them, somehow had clean and dry hair and a charged and dry iPhone after 8 days in a rainforest, then somehow died violently enough their bones split and wound up many miles away up-river" isn't the simplest solution.
"Were accidentally or deliberately killed in an area rife with violence where murder is not uncommon" IS the simplest explanation in this case. The strangled American tourist and the kidnapped German tourist both happened in the same area; if it could happen to them, why not to the Dutch girls?
given the sheer unlikeliness if someone fiddling with the camera etc then
I don't think it's unlikely. The tour guide had a key to the girls' room where the camera charger/lead was, and there were computers there. It would not have been hard for him or another local to delete the photo. They girls weren't in some deep wild jungle, they were a couple of hours easy walk from the town, on a trail the locals used every day. It would have been the easiest thing in the world for a local to pop back to their room with the camera and delete the photo. There's also a theory (from someone alleging - emphasis on alleging they have evidence) it was deleted by local police; a bit outlandish, but given the level of corruption and fact tourists have reported the Panama police covering up rapes, not impossible.
If you haven't followed the details of the case, it's easy to assume the existence of a wild river, or steep ravines, or thick disorienting jungle where you get lost if you stop off the trail, or man-eating animals, or whatever. But in reality these things simply were not present or have been ruled out.