I would drive the car to another shop; Go to an independent shop,
not a dealer, that specializes in European cars. They should be able to scan the car and get a DTC Diagnostic code. Have them scan it as a 2003 Mercedes SLK 320. It is a R170 platform. There is a list of repair shops that others have used here on the forum and you may find one near you. Click:
repair shops
If they can't find any codes then you probably have a bad RCM or the fuses on it are bad, or the wiring connections on it are bad.
There are so many possible problems that cause the issues you are having, but they will throw a DTC; No DTC = RCM
I am not convinced that all the items listed are Good. You apparently haven't tested them, but are taking the mechanics word; so much can be lost in that conversation. This could be something as simple as the brake light switch, but that will throw a code. However if the cruise control works then the brake light switch isn't causing your problem.
Two things come to mind: 1) under the carpet where the passenger feet would be, pull back the carpet, remove the stryofoam and the steel plate. Mounted to the steel plate is the TCU, radio amplifier and several other modules. Inspect that area for water intrusion and corrosive damage to the electrical connections.
2) start pulling fuses until the lights on the dash go out. That will at least narrow it down to what circuit could be causing all this trouble. There are fuses on the RCM to start with and then replace that fuse that extinguishes the lights, and proceed to the fuse box under the hood drivers side; If that doesn't do it then there are fuses on the end of the dash drivers side.
I can't think of any other way to proceed; Hope you figure this out! Please lets us know what it took to get your car right.