the ECU will detect misfires and store them - in any case - whether its missing fuel, spark or compression. The ECU actually is very closely monitoring the acceleration of the crank position sensor as each combustion event takes place.
If you are seeing a cyl 5 misfire, then start with seeing if its actually a cyl 5 misfire - the reason to mention this is because when a crank sensor is replaced on a mercedes M112 engine, the ECU needs a relearn command from the SDS tool to "initialize" it - it basically goes into ultra sensitive mode and the ecu commands some odd events, this makes the ECU learn each cylinder firing pulse again which is used for such error codes. - Its really a great feature as compared to toyota which will only throw a cyl misfire code when a coil goes bunk.
You need to test all parameters - exchange coils with cyl 6 and then retest. Test the spark plug cables and make sure they are seated all the way in. The connector on the coil should be good too. and good spark plugs are mandatory, no fancy 20 tip unobtanium items, simple NGK or Bosch platinum plugs
if this passes then test fuel pressure and do an injector balance test. If it still does not work - then do a compression test. If you need help in how to do it, I can help you - (you dont use the ignition key to do the compression test on a mercedes M112 engine)
What other errors were logged in - you mention gear module - did your tech not test/interrogate the gearbox control unit? There is a huge number of very defined error codes that can be drawn out - which makes repairing a walk in the park.
The gearbox shifting at 2100 rpm is not any sort of test or indication. If you floor the throttle it should shift at 6000 rpm, the shift is controlled by actually measuring the engine output torque (input to gearbox) and then gearbox output torque. If your accelerator pedal position and engine load parameters are not demanding, the gearbox will shift into higher gear early to save fuel.
by default the shift style of a mercedes 722.6 automatic is very aggressive. it will kick the car on every gear and every shift will happen sharply. It slowly learns your style, if you drive like a business chauffer it will learn to shift softly.
