Sounds like you got screwed.
This happened to me before so dont be too upset. I know how you feel, trust me I do. I spent about $1500.00 in car repairs and my car didnt start still. I told the service advisor I will not pay you $1500.00 and tow my car back home.
Anyway, you would think a state of the art MB dealership would have testing equipment, code readers and a lot of knowledge. You and I can do a plug and pray repair job, why take it to them?