Rev power is high even whilst on parking mode

Hello vannafrangos,
Was it an authorized Mercedes garage you went to?  Or was it an Independent garage?  It should be in the capabilities of an authorized garage to identify the problem if they had, had connected up to the Star diagnosis as is customary.  The diagnostic machine should have given the technicians some fault codes from which to further identify the problem.

Without those codes, I can only guess at the likely issues that give you, your faults.  Your coolant temperature sensor could be reading falsely.  The temperature sensor sending signals to the ECU could be different to the one sending signal to your temperature gauge.  Does the temperature gauge read correctly or the temperature shows cooler than it ought to.  The idle speed sensor is faulty. The engine load control valve faulty, the coolant thermostat stuck open (resulting in slower engine temperature warm up)......
Throttle switch or throttle valve.
Cheers,