Short answer to your first question, yes.

One supplier admitted they substituted sedan rotors. The dealer and the parts store I got the Zimms from both provided what were supposed to be 'updated' part number items.

As Zinn is the OE supplier, one would expect at least them to be in their game.

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.

Have you looked at the part # of the disk removed?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

The discs were 'white box' eBay specials put on by the PO. No part number.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22