1) Yes, order a new RCM and put it in yourself. (They literally plug in, without ANY tools, altho a flat blade screwdriver helps make it easy to unplug the connectors from the old one).
Then mail the bad RCM to DJ and have him rework it, that way, you have a spare.

Now, it may not be the RCM, it may be the Crank sensor - you did not indicate if, when the car dies and won't start, whether or not the starter engages. If the starter engages but the car won't start, it can be the RCM or the Crank sensor. If the car won't even engage the starter, it's the RCM.

Id' order a crank sensor as well, in fact I'd order two, the Crank sensor is a part you should also have a spare for - I sure do.


Here is the RCM, order it today:
https://www.summitracing.com/parts/s...AaAoMDEALw_wcB

Crank Sensor - order the BOSCH at the bottom of the page (if you order the Bremi or the 'OEM" part, please don't ask for help here ever again):
https://www.germanautosupply.com/cat...nkshaft.Sensor

2) Stop taking the car to a "mechanic" and expecting him to fix ANYTHING. There is no factory support for the Crossfire, if the guy does not know that it's an SLK320, he will be lost. If he does know it's an SLK320 but has no experience with them, he is going to be lost.