It might be sensible, though, to couple a killswitch to the handbrake, so that engaging it switches the engine off, or cuts off all electronic control, or something.

I don't drive cars, so i have no idea if that would conflict with normal uses of the handbrake. Perhaps it could have a position beyond the normal brake-engaging position that did this? So that if someone panics and yanks on it as hard as they can, they get the result they probably want.