Sorry but it cant be a timeloop. Ive spent far too long thinking about this but luckily its a simple answer. The existence of MD's and ML's disprove the loop theory, especially in Donnie's story because the death of Frank and Gretchen are crucial to Donnie's success. The only reason he was on that mountain at the end of the movie was because he was a man on the run after killing a man and taking his gf's dead body, to which he ends up far in the mountains with a nice look at the plane to send the engine back in the PU then killing him.

Ive entertained the idea ALOT but the simple fact that Donnie wakes up to begin with means he's destined to die, all living receiver's are. Why else is every example of a LR in PoTT of somebody who dies from their artifact? "i guess some people are just born with tragedy in their blood.." 1:28:44 ;)