In the full day at the dealer with the higher-up MB service reps., did anyone suggest swapping in a new alternator as a test? I'm not a fan of throwing parts at a problem, but after one FULL day of diagnosis at a dealer service facility with NO conclusion, I'd be trying a new/different alternator. One failed diode in the triad?...sextad?...whatever little "loop" circuit is internal to the alt., could be your problem. Bosch rebuilt for your car.. less than $200 at Fastlane. Reliable and non-annoying charging system..priceless
FWIW