Does your 06 model uses Drive by Wire system? You can check the throttle to see if there is a cable goes into the cockpit, if not then you have DBW system.
DBW uses two voltage sensors by the gas pedal that communicate with ECM and the ECM controls the throttle blade with steps-motor for fuel economy. Normally if one sensor fails the 2nd one takes place as a fail-safe mode, it's rare to have both goes out at the same time, but it's very common on Mercedes cars.
Now I don't know the voltage spec for the pedal sensors, however; you can create another ground by poking a needle behind the harness where the weather seal is on the Negative side and clamp a lead on the needle and clamp the other end to a good clean ground near by and see if that works. If it does then your sensors are good and there maybe the problem on the ground side between the sensor and ECM. Otherwise before replacing the sensor, you should find out the voltage spec and check the + terminal and sweep the pedal (press the pedal down slowly) to see if there is voltage hick-ups somewhere when you depress the pedal. If the voltage readings are good to spec, I would suspect the throttle blade's step motor on your intake manifold. I hope this make sense.
I'm just a student learning mechanic at the moment and I can't believe the dealer tech couldn't solve your issues. You should complain to the manager or Head office, don't let them keep charging you. I'll check at school to see if there is any recalls for your model regarding the DBW system.