"Where do players fit into the story of the WoW? For example: Eventually players will be able to kill Arthas. How will that be written in the stories?"
Players are the heroes! Grab your sword and fight the Horde! I often find it helpful to think of players as the "reader" or the "third person". You're reading an epic story told via your game. Of course, that's for events that unfold infront of you.
For events that you're a part of, like the death of Illidan, you're usually at the very end of the story. Illidan's transformation from Malfy's "good" brother to the evil tyrant he became is told in various stories. You know going into the story that he's going to die at the end. Who kills him? Well, you do! The journey of how things progress to the point where you get to kill Illidan are must more interesting than the actual act of killing him.
I'm using Illidan in this example because we don't really know what's going to happen with Arthas. We know we're going to face him, but we might not kill him.
"In the Argent Tournament, why does the Night Elf quartermaster sell a gun instead of a bow? Night Elves don't typically use guns."
All items are the same across all the quartermasters, except for the mount, tabard, reputation token, and non-combat pet.
"Blizzard's site said Noblegarden was based on Druidic traditions. Assumedly like Wintervale... So um, what exactly is up with those Druidic traditions?"
One could surmise that it's because the Druids / Night Elves are the oldest original race still active in Azorth's day-to-day struggles, and their traditions are thus the ones that are most prevalent in world. The Aspects are rather removed from events, and Orcs are not part of the original Azeroth. Every other race is younger than the Night Elves, with the possible exception of the Trolls, but we don't have enough information on them.
You could also say that most people outside of the game have a different definition of what a Druid is (well, somewhat different definition), and that it allows more people to easily relate to in-game events.
My guess is both answers are correct.