Hi Makpatti,
In case you tried to disengage the shifter when it was working correctly, start up the engine, steer the wheel to the left until it does not move further. Hold it there for ten seconds. Then steer to the right and hold it there for ten seconds. Turn off the engine. Restart the engine. The shifter should work properly. I am far away from Dallas in another state, thus cannot recommend you repair shop. However, the shop should never laugh at customers who pays for their living. Before you choose a shop think twice; are they going to be able to fix your car or damaging it because of the lack of understanding a more sophisticated car like yours? Therefore, ultimately the MB dealer’s shop should not be tossed out from your plan. Dallas might be a big enough town though that you could find an independent MB repair shop.