Obvious explanation for the visions: GUILT and hallucinations
SPOILERS
Even the kid Billie whom apparently attacks Nelson.
They all stemmed from their respective guilts, and that's when upon addressing them and solving the conflict, the visions stopped.
And it didn't matter whether the person involved/affected was alive or dead. It only mattered that the haunted person FELT relieved.
So they were all hallucinating, plain and simple. No ghosts were attacking them in any way.
As for Nelson being attacked, the movie clearly shows it was his own doing. In the truck when "attacked", LabRassio clearly sees him struggling alone and with his eyes closed (clearly not fully conscious, as in a dream or a trance), and when he intervenes and shakes him awake and wrestles the ice axe from his hands, it's obvious there's no invisible force at play (otherwise LaBrassio would have felt it or even been attacked himself).