Why couldn't you just hit the ignition to turn the engine off?
http://www.worldwidemotors.com/blog/how-to-use-push-button-s...
Personally, I'm concerned there is no manual brake override.
If I drove a Mercedes I'd test using manual ignition, maybe pulling the key out would stop the car.
throw the key out the window?
Source: Don’t rest your hand in the center console on the freeway.
You can also just shut the engine down with the ignition switch.
The brake pedal is alway mechanically connected to a split/redundant hydraulic braking system.