Plot Hole: If Marie and the killer are the same person, how can she be in the house/gas station parking lot when the killer is in the cornfield/speeding away?
Possibly her perceptions of time are distorted, so she can do certain things as the killer and then react to them as Marie. But still, where did that truck come from?!
Marie could reasonably have acquired the truck and other equipment she needed before the film started and parked it somewhere nearby.
Another one that is actually a physical impossibility: Marie is a woman, and therefore lacks certain body parts. If she is the killer, then how does "he" fellate himself with the dead girl's head or implicitly rape Alex? Also, when Marie and Alex are locked in the back of the truck... WHO'S DRIVING?!?
Having not seen the movie, she does have fingers.
If she manged to imagine the killer as a separate entity from herself it is logical that she would be able to make herself believe other things as well. Having had the ending spoiled before I saw this flick, I assumed that the fellato-scene was part of her imagination (i.e. not actually happening) and meant to visualize her creating/setting up the murderer-persona. The truck being in motion can also reasonably be something she forced herself to believe, while in reality she just stopped the truck occasionally to check on Alex. Hell, if I remember correctly there was not even anything suggesting that the truck actually was in motion during those scenes, apart from the editing.
Never quite seen the problem with things not quite tying up or making total sense; the story is told from the point of view of someone who is revealed at the end to be completely cloudcookoo-land mental; by their very nature an extremely unreliable narrator