As for the gun part, "Greg" couldn't have shot himself. He grabbed the 9mm pack, looked inside, took one 9mm handgun out, threw it on the floor.
After having been hit by Sheppard's P90, he fell with his hand next to the 9mm. He takes it, then aims (quite well, might I add) at Sheppard, shooting and missing, until he hits Shep. in the arm.
Guns ARE hard to handle. For someone who doesn't know anything about terran technology, he learned QUITE fast how to use a handgun accurately.
As for Weir, Teyla, and Ford... Well, this WASN'T their episode. The episode was made to develop Sheppard and McKay. And I must say, it was very well done. McKay's arrogance was shot down by the constant waiting in the supply ship. Not to mention, his arrogance was thrown aside when he faced "Greg" one on one.