Dealer suggested I call the biggest local Indy Mercedes shop to see if they have run into this.

Answer yes, but not specific to this model and never with both sides at the same time.

I live in an area that not only sees major use of road salt in winter, but high humidity year round. The Indy says he has had cars where the backing plate swelled or deformed from rust, and a newly installed rotor wouldn't turn.

He suggested grinding the lip down on one backing plate as an experiment.

Other than that, he is suspicious the backing plates have been changed (simply because the ones in there now aren't crumbling away) and the sedan ones put on in error. He says mistakes like that sometimes work until rust comes into the picture. Anyway, my wrench is adamant they were never done.