Looking over this and your previous post, and having the understanding that actual ECU failures are rare, I would ask the following:

1) Were there any symptoms or codes before the car stopped running? Did it just quit suddenly while driving, or not start one day when it ran ok before?

2) If you return the factory original ECU to each of your cars does the other car (from which you borrowed an ECU) no longer run when it ran ok before you removed that ECU? I am thinking that a car might refuse to run unless the ECU is absolutely a perfect match (or has only a few specific benign differences).

3) Did either car experience an overvoltage condition - from an open voltage regulator, or use of a high voltage jump starting machine some time in the past? I ask because overvoltage is one way to damage the ECU for sure. I think many of the inputs and outputs are short-circuit and open-circuit protected because a designer would anticipate possible failures on harness or sensors, as you seem to have had or suspect.

4) I don't have schematics at hand, but do these cars have a main relay or fuel system relay (not the fuel pump relay) which passes power to ECU and/or primary ignition? Did you verify it is ok?

5) Do the injectors tick? If they do that suggests that some functions in the ECU are intact.

good luck!