Many years ago, Gene Holtzclaw had similar problem.
This must be one of the longest threads on Marelli no-start problems. Here:
While I agree with everyone who says that the first suspect is the CPS, if the history of the car is unknown and someone else has messed with it, ohming wiring is good but not sufficient.
As you can see in the discussion with over 250 replies, Gene finally solved the problem by discovering that whoever worked on his car before, tried to repair the engine harness. In the process, s/he reversed the polarity of the CPS connector (next to the B-bank thermostat housing).
I think you need to check that connector and rule out the possibility that signal wires are shorted or reversed with the ground wire.
To repeat what others in this discussion already stated – the ONLY way a Marelli car will not start (I trust you’ve checked fuel delivery and have confirmed that spraying starting fluid in the intakes does not change anything) is if the ECU does not get signal from the CPS and/or flywheel sensors.
Good luck!