Usually, the dealers are horrible with these cars (unless you can find a Mercedes dealer that will admit the car is a Mercedes and have his shop work on it).

I believe the SKREEM is the problem, but I will just throw this out there: This is very similar to what my Durango did one time. We had it TOWED and they worked for an hour on it - then some guy said, "Put a new battery in it". We traded batteries between the Durango and my van - it started! Put the original battery back in - no start. (BOTH batteries started the van!)

Older batteries can cause weird stuff in our cars - and some others.
Just saying.... but I still think it's the SKREEM.