CoG is for the entire vehicle with whatever is added at the time you pressed O. So can include driver, passengers, fuel and mass handicap. When local Z is zero, you are still at the reference height (in world space) above the real ground. This needs to be added to the local Z to get the true height.
The X position is very close to zero, I'm not sure why it isn't exact. You'll notice that even with the driver out, road cars are heavier on whichever side the exhaust is. This isn't tied to the model directly, but the numbers used to generate the CoG location may well be generated from the model at the design stage.
I think the car body aero lift and drag are always applied at the unladen (i.e. no driver or fuel) CoG. So there is normally no weight transfer from aero drag.
Who cares where the world co-ordinates origin is? Somewhere below the car.
No idea what the body matrix is.