Some preliminary conclusions:

- "A and B [...] work a lot better individually." So they probably separated while collecting firewood.

- "A has OCD and can't stand things being messy. [...] There's a pile of firewood at the campsite, piled extremely neatly." So the firewood that's already there was probably brought by A.

- "There's firewood under [B's] body, suggesting that he was carrying it at the time of death and fell on top of it after getting hit." B was only just bringing his firewood to the campsite when he was attacked.

- "E and F [...] pretty much can't lift more than their backpacks," so it doesn't look like either of them was the one to hit B with a chunk of wood hard enough to kill him.

- The campsite bears "no major signs of struggle", so we can assume B was killed there and C and D were knocked out there.

- A and B are best friends, so we can assume neither of them killed each other.

- By the logic now stated in the latest edit to the question, G did not kill B.

We've now eliminated A, E, F, G, and presumably yourself (unless you're an [unreliable narrator](http://en.wikipedia.org/wiki/Unreliable_narrator)) as B's killer, leaving only C and D. Since they were together the whole time, they must have conspired together to kill B.

By the same logic that eliminated E and F as B's killers, they couldn't have knocked out the "jack[ed] up" C and D. This was probably done by A, who has the best motive as he was B's best friend - perhaps he saw them kill B? A could then have been killed accidentally by his raspberry allergy: when he attacked C and D, he was so overcome with grief and rage that he failed to notice the berry stains in their hair until it was too late.

Apologies for the lack of spoiler tags; let me know if you want me to put some in!