It certainly is easy to try a new fuel pump relay - it would be a good first step. It would be really nice if you could try a fuel pressure test when the car was not starting. This would verify the fuel system was delivering fuel.
I forgot that the 400E has so many systems that must be checked. Its kind of funny since all these systems are interfaced via a databus. You would think that it would be possible to access all the codes together. Maybe that is what the diagnostic module is for....I'm way over my head here.
As I recall all the codes can be read and reset using a standard impulse counter but its not quite as easy as the K-jet systems because the impulse counter has to be hooked up to a 38 pin diagnostic connector (and the pins on that connector are mighty small!!) There are very nice breakout mushroom adaptors that make access to individual pins easy but the connector is a couple hundred dollars. I don't have the diagnostic manuals for the M119 engine and a quick check of my 124 service CD rom does not seem to detail the procedure for code readout. Maybe somebody here could direct you to a source for this info.