Benny snuck up on Courier six and shot him in the head. He had an unfair advantage.
Hang on a second... In the opening scenes, isn't the courier tied up and unconscious but not yet shot? Sounds like he got jumped and knocked out. He got shot later.
It's a strange scenario, anyway--
Not only did Victor dig you up, he carefully excavated the entire grave. Huh? (obsessive robot behavior, maybe?)
A schmoozing butt-head and a gang of Khans take the time to
bury someone? In a graveyard?
I'm not amazed that the courier survived a headshot; hell, my couriers get shot in the head all the time. All Doc Mitchell needed was a Stimpack or two, right?
What does House see in the Courier? Obviously he's in cahoots with Doc Mitchell, who hypnotizes you with Rorschach blots, inane questions and possible brain alteration to seek out Benny.
As for the other factions enlisting you, I got nothing except that it sort of makes sense for the NCR to seek your help because by the time you reach Vegas you've probably helped them in some way. Can't swing a non-existent irradiated cat in this game without hitting an NCR quest.
Not an attack on NV, and kidding.