Well, your cruise control can still work fine, and the actuator could still be bad.
The point is that if any of the three functions the actuator controls fails, then you have to replace the whole part.
I agree with you in that you need a mechanic who specializes in Mercedes to diagnose the car, whether at a dealer or an independent.
Two Mercedes independent shops that tried to fix my car couldn't find the vacum leak. They suspected it, but determined that there wasn't one.
The dealer took less than an hour to determine that there was a vacum leak, and that it was coming from the intake manifold seal.
In any event, the throttle actuators are a known failure item on our car, especially because the early ones (like ours) had bad harnesses.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".