So you had a bad RCM that was causing the no-start and some asshat company screwed up a perfectly good ECU/PCM.
This was an easy fix, had you come to this forum first. You might talk to Josh at the MSS, if he can't help you, it would make a good parts car.

Call this place and ask for Josh:
https://themercedesswapshop.com/
Do what he tells you and do not second guess him, he is THE MAN for this. He and Rob at Needswings are the two top-tier vendors for our cars.