Tough one to know who to call exactly, as you may or may not know, until recently I was a dealer tech.
I'd say what needs to be done is that you need to speak to the Zone Manager for your area. The easiest source for this info would be the dealer, assuming they cooperate. Often the salesman who sold you the car may be the most eager to help, as the salesman will realize that your satisfaction depends on matters like this, and may be actually more helpful than the service dept in getting it resolved. The salesman wants to sell you another car in a few years, it's hard to get the service department to think that way.
If you have no luck at the dealer, you could try the 1 800 For Mercedes number and ask if they can get you in touch with the appropriate Zone Manager for your area.
If they ask if you want to talk to the District Sales Manager or the Service and Parts Operations Manager, I'd say the slaes and parts guy (SPOM), but either of them should be able to help.
In the meantime, look over your warranty book and see if it says anything about this 12 month/12,000 mile warranty for wear items like belt, hoses, bulbs, brake pads, stuff like that. I still think you can get them to give the money back for the brakes, that wasn't right, at least not if you are under 12month, 12,000 miles.