Fuel pressure won'y set a code right away.
I don't know what the diagnostic computer was telling them, but I take what they say with a grain of salt anyway. There is no such thing as a diagnostic answer from a diagnostic computer. It is only guidance.
Without a problem when the diagnostic is run, all one has is a few hints from the records of failure. No fault code is taken for certain. It is the basis for testing. If the problem is gone, substitution becomes the diagnostic test plan. They have so far eliminated 3 items from the possible problem causes. When they get the problem fixed I would suggest they replace the old parts that obviously don't contribute to the problem.
Bosch Master, ASE Master, L1
33 years MB technician