You mechanic seems to be on top, but you have one of those super aggravating intermittant problems. Unless some security device is intermittantly causing an ignition interuption, I would do this with your mechanics help. Install a T fitting on the output side of your fuel filter and hook up a good quality fuel guage to it. Run the line into the trunk and make sure nothing is going to chaff the hose during normal riding conditions. When the car fails to start, or stalls, go see what the fuel pressure reading is during cranking. Naturally, if the fuel pressure is off spec during these failures, and then when you eventually see the pressure rise and the car starts again, you need a fuel delivery overhaul. Fuel Tank cleaning or draining, a new pickup screen, main filter, fuel accumulator, and fuel pressure regulator. An ignition failure can be seen by immediately connecting a spare spark plug with its side electrode cut off, and remove the most closest high tension lead from engine to plug the test plug into. Let the plug body lay accross some large metalic area of the engine, and keep the sunlight out of the area to see the plug firing. Dont ever hold the lead in your hand!!!! The reason we cut of the side electrode is too see how powerful the spark is. If you see no spark, and considering all else that was changed, you might have a intermittant connection to your ign coil, or a weak coil. Clean up all connectors on your EZL module too. (if you have one) I would put my money on a fuel sys problem though, the MB ign sys is very reliable.....
PS: The fuel pump relay is probably incorperated into the MAS Module. The module can be opened to visually observe the relay energizing, but this requires skill to avoid accidental shorts or damage. Some mechanics clean the relay and bend the moving contact to get it working well again. Its the MAS module that needs an exchange to see if its a fuel pump power delivery problem. Check grounds on the fuel pumps.