You could have a fault in the idle speed circut but your symptoms still lead me to believe that its a throttle linkage lubrication issue - I've had multiple cases of first hand experience with the exact problems you are describing and all were caused by binding throttle linkage pivot points.

If you want to test your theory you can disconnect the ELR actuator on the IP, this will defeat the idle speed control circuts on the car and it should idle (roughly) at around 500-550 RPM.

If there is indeed an idle speed circut malfuction I don't think the amount of adjustment capable in the system could possibly result in a "runaway" condition and I've never heard of an OM603 suffering from this condition.

Even if your car did suffer a runaway condition the braking system has more than enough capacity to overcome the engine and stop the car. If you don't believe me find a nice empty parking lot and try to move the car with the engine while you stand on the brakes.

You may want to check the condition of your flex discs before your try this....