For what it's worth there is another option you might try. You stated that you have been having a factory dealership work with the problem without coming to an acceptable answer. The factory does have a team of technical specialists working for the respective zone offices, at least here in So Cal. They usually work with resolving technical issues in newer models but have been known to assist with diagnostic situations when requested. You could have a meeting with the service manager of the dealership you do business with and see what can be done to help resolve the issue at hand. You can also contact the zone service manager with Mercedes Benz for the area you are in to see if you could get help in resolving a technical issue in an MB product. I would be surprised if you didn't get some technical assistance through the dealership. The assistance discussed would only be available through the dealer though. Just another possibility.
Bill