What are the possibilities of a French speaking person answering Theo's texts pretending to be him? Say something happened to Theo in town, an argument with another French speaking tourist which went wrong? How long was Theo at the end of Tennyson? Pretty sure that he stopped there for a bit, and even a few minutes is enough to land in serious trouble. So say an accident happened as a result of a scuffle, the perpetrator panics, Theo's phone knowing it will lead to there, panics and runs all the way to Cosy Corner, takes the bush route to avoid being seen. Stops at Cosy and rests and uses Theo's phone answering his texts, even watch part of a video, make it look like all was well up to there. Using maps en route to make it appear he was lost. There are plenty French speaking back packers around and someone who was in serious trouble would pick the bush route over known trails to avoid bumping into anyone, to make it appear Theo was lost maybe. And would kind of fit that this person was in a hurry. It is a fair way to run without a good reason. Someone in a panicked state would move fast.
If this was the case then where would Theo be? If this person had a vehicle nearby then yes could have hidden him while he/they take the phone away. Later the hat after being cleaned is planted along the same route. The only confusion with this theory is the phone pinging next day. Why would they not dispose of the phone? Possibly left somewhere very well hidden, inside a hollow tree or similar, hoping the signal leads to the area, for all they would know people could have been searching next day so if his phone was pinging there, well then that is where everyone would look. Sounds way out I know but is it possible? I think the chance of this happening is there.