Question: When Kimberly's friends died first in her vision she and her friends died last, but why were they killed before everyone in reality?
Answer: I'm not sure of a very logical explanation for this, but it should be noted that the premonitions in the films are not 100% accurate as the accidents are not caused by Death itself and may not be based on reality. They are just premonitions that the characters have of an impending accident that happens due to some unfortunate circumstances such as malfunction or improper precautions. In short, It is merely Death's job to clean up those who manage to cheat death in these misfortunes. He plays no part in causing the accident itself.
Then why do several characters talk about Death stalking them? Or the fact that the chances of somebody strangling themselves in a bathtub, for example, are one in a million? Or what about Death's design, the signs, and Death's list?
Question: Why did the boy at the end die? At the barbecue, they tell the story of how Rory saved the boy from the news van and realise since Rory intervened when he wasn't supposed to be alive, the boy is on the list. But if all of them had died in the original pile up, they would not have crashed in the field, the news van wouldn't have been there, so the boy wouldn't have been in danger.
Answer: As stated in this film the survivors of the highway crash were all supposed to be somewhere where they would have died, but the flight survivors started a chain reaction that prevented their deaths. We could assume that the boy was also supposed to die at some point but his death was intervened once before and again when Rory saved him. So "Death" was still catching up.
Answer: Kimberly and her friends died last in the premonition. By not getting on the highway, they're cheating death already. The diesel ends up on its way to kill Kimberly and her friends, but the cop pulls Kimberly out of the way, causing her to cheat death again.