you can always try for some more help at www.mbca.org
or www.benzworld.org
and maybe a different Mercedes dealer or maybe try to make an appointment at a dealer with the regional rep.

After the dealer worked on the car and it went bad in no time you should have gone back to that dealer instead of taking it to another shop.

If you attempt to sue that independent shop the cost and time may not be worth it, have you tried to get your money back from the independent shop?

Also if you go back to a Mercedes dealer try to talk to the shop foreman before any work is done. Explain to the foreman everything that happened so hopefully he'll oversee any repairs.