Not sure RE the button, especially if an auto in drive at the time.
However to answer the what if you passed out with your foot pinned, a passenger can do the same as in a manual car and knock it into N and then use the handbrake gently to bring the car to a stop. Putting it into N will stop drive to the wheels allowing the car to slow down regardless of what your unconscious leg is doing on the throttle. Also obviously care would need to be taken that the rear wheels dont lock when applying the hand brake, although at that speed they would need to really wrench the hand brake lever to brake the rear wheels lock. In the newer models with a button for a hand brake if you lift and hold the car will apply the hand brake regardless of speed, however you are much more likely to lock the wheels so better, if there is room, to knock into N and let the car roll to a stop, applying the parking brake when very nearly or completely stopped.
In reality, in a car with a key ignition, the passenger would have been unlikely to be able to lean round the other side of the steering column and turn the key anyway so even if pushing the start/stop button does nothing (which i suspect it would) then you are no worse off than pre start stop button anyway
