best explanation i have for what happened
my theory about what happened is that he was in a coma (even when he was told at the beginning he had been clinically dead) and what made him wake up was finally accepting his own death. the accident only magnified his giult that he was with his brother's fiancee to the point of making himself wish he was dead (subconsciously). the emotional shock of the acident only triggered a state where his mind thought he was dead. so his own death in the accident was only a psichological response, somehow making things right for him, deep down. so, in the end, it's all just a dream, the way his own mind finally works its way to get rid of the guilt.
jumping from one place and time to another is explainable somehow for the mind by delusions and loss of memory which he cannot control, but the true peak of the nightmare is when his brother tells him the truth: that he is dead. this is whe he finally wakes up, for real. in this way, it is very significant that it is his brother who tells him he is dead.