are you able to check much yourself?
Worth checking the earth's are good, including the earth strap from the battery is properly grounding at both ends. Seems odd that the sensor is reading high on 2 different ECU's, and being a new sensor that pretty much leaves the wiring/switch-gear as common elements. Are you getting a good 12V reading off the electrics, say at the relays in front of the battery, with the ignition on?
Sounds like it'd be worth investing in a reasonable digital multimeter rather than fork out for the garage to continue, if you plan on keeping the car. Are you using an Alfa specialist or a general garage/dealer? Did they try running a direct wire from the sensor to the ECU and check continuity/resistance on the existing wire?
Funnily enough, I've a Y2000 2.5v6 Q-system saloon that will be needing a new home, as I've 2 of them at the moment, but may have a buyer for it, just waiting on a call to arrange something...