>>> Are my missing something? <<<

I don't know is the short answer, but I always try to work through the options.

The central electronics unit has the job of controlling the fuel pump.

You said the pump runs for a couple of seconds when you first turn on the Ignition and then runs when the engine is running.(which is what it should do)

Question: how does the central electronics unit "KNOW" the engine is running?

What would happen if it lost this signal? or was "TOLD" to switch it off for some reason? (Perhaps the result of the ECU monitoring function)

I think you MAY be seeing the "effect" and not the cause.

Bosch MV diesel Injectors normally work between 70-90 volts, but you are saying that the wiring diagram shows them being supplied via the same relay?

Maybe nothing, but its just the way my brain works (or not as the case maybe)

Mark