Engine won't start. Car makes noise. I understand your position about the two computers. Looks as though I had them backwards. I'm not even sure what he means by "limp mode" guess it was something he came up with( I don't know). Maybe just car not working.
Modern cars sometimes have a "limp mode," so it is a thing. But I don't believe that the Bosch L-Jet system in your Spider has anything like that. (And I don't believe that there is any computer diagnostic system that can be run on the L-Jet computers). I'm concerned that your mechanic may be a bit over his head in dealing with the problem. There are many individual tests for the various sensors that are the inputs to the L-Jet system. Normally, the problem is not either of the computers, but rather, one or more of the sensors (or fuel pump, or relays, or something else). My suggestion would be to get the car back from the mechanic and step through the L-Jet diagnostics page here
to see if you can diagnose the problem yourself.
'85 Spider Veloce
'17 Giulia Ti Sport