He is fully responsible for your car ones you drop it off at his garage for repair. That is upto him and his insurance agent now.
First, if he didn't touch the engine (like oil change or sensor replacement) at the time of tranny fluid replacement, your engine should be fine.
It's not easy to take out transmission and install it back in your car, it would take a good day (from the description of your mechanic, I would say a week but that is just my opinion). Check is your CPS (crankshaft position sensor is not unpluged or damaged). That could be why you are having problem to start it up and this is the closest thing to transmission on your engine (behind driver's side cylinder head). No signal, no start otherwise big engine damage will hapen.
I just bought used transmission from junkyard last year for $400 in the case my brand newone will quit but so far so good with 25K on it. For $3,000 you can buy whole car with body damage on any dealer or insurance auto auction.
I would tow it back to your professional mechanic and have him call me when the car is done or in the same condition as it was before he start working on it! It should not be your resposibility where and for how much he will find used tranny to fit your car.
Tell him to pay for your rental car or give you a loaner car by the time he is taking care of your Mercedes.
Now, when you attaching transmission back to engine, it needs to be perfectly lined up in order to turn the engine freely later. Flywheel is attached to torque convertor through flex plate with only 6 screws. you need 13mm socket to take them out (small access hole under plastic cover) and your transmission and engine are two diferent things without any effect to eachother. Your mechanic should know this, I'm not one but I know and I'm sure that he didn't have your transmission out of your car just to turn your engine by hand. I hope he is not trying to charge you for this kind of work?!

 

It is not battery problem because if it is, the starter would not click at all. When you shift to P, can the car move? If it can, this indi did not install your tran back properly. It could be the alternater problem only. Take out the alternater, put it to the 12V battery and see if it works. If it does, then it is not the alternator. Then you should bring it to another indi for a second opinion.

 

I totally agree with MBenz80, the only problem is that once you took the car out, they would say the responsibility is back to you, and it is difficult to argue. If they try to con you in the first place, you cannot trust them any longer. DO update us.

 

Hi there i think I know whats wrong i did have same problem after fluid change, 2 blocks and tranny is out this is more easy then You think , please check the oil in the engine red dipstic , let me know . MD

 

Will oil level sensor protect the engine from turning, if sensor is bad and ECM thinks that you have no oil in your engine? This is interesting, same oil level sensor they are now installing on most small engine motors to protecting running them without enough oil in them. Dealer should know right a way what was wrong with that car but they were hoping for your $12,000 check.
Bernard

 

This is haw the problems start. Transmission oil change mechanic drain the oil change the filter and then they put 4L oil in to the engine thru the dipstick , because is red and on the back of the engine, the engine is full of oil, and the transmission empty. after they change the oil, and do not add any to trans. there is some oil left in torque converter to go for couple miles, I fix 3 cars like this. Mark