Wing would have to be full wing, as I've flown an F4U a couple times with half of one wing shot off.
Pilot, obviously, although I think that's already automatically a kill.
I think if the engine is shot out the damage count should stop, but NOT be automatically rung up as a kill, unless someone else actually shoots the plane into the ground. The guy who hit the engine still gets the kill (as he took the plane out of the fight). However if the pilot manages to glide to safety or nurse a crippled engine home no kill is recorded (which is where it stands now, I believe).