That is not an air metering screw, it is a FUEL metering screw. I would turn it back. 1/4 turn is enough to stall the engine. We usually only have to adjust it about 1/8 either way to achieve the correct fuel mixture after a service.
One thing I have seen is the ignition coil fail like this causing a weird color on the spark.
Yes, you do have a crank sensor, but when it fails, you have no spark period.
Partsshop can send you any parts you need. You may just want to go to the Fast lane and open the Online catalogue and look around.
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MERCEDES SPECIALIST 11 YRS