0.5 volt is the correct voltage for a non running engine, when you crank it the voltage should rise as the pressure does, if the computer does not see the correct pressure it will not fire the injectors.
You can not trust your scanner so i would go back to basics and make sure the fuel filter is full up and make sure the fuel line to the pump is also full, not sure if the merc has a low side pump in the tank some do some do not, i like to look at wiring diagrames to establish this if it has got a pump in the tank you can power the fuse or jump the relay to make sure you have fuel in the line or . use a electric pump on the fuel line to the pump and suck up the fuel from the tank , and once you have a full filter and a solid supply to the filter , the low side pump in the engine bay will be able to do its job ( if its got one).

do this and a few whiffs if easy start will get a diesel that has run out of fuel started right up, if thats the only problem.