ECU's are identical on the outside. I'm curious how a later Regina ECU, with a different number (I also have such a spare) would work in my 1990, but that's a different experiment.

I might get ambitious and try reversing the fix, but it's actually more of a project than it might seem.
This experiment would have to take place in the snow in my -5F driveway where there would also be wind to make it more cheerful.
I need to disconnect and reconnect the battery and unset all my radio presets; again.
Then I can do the trim and ECU removal and replacement while kneeling in the snow.

Then, presuming it really does not want to take any throttle the next time I start it, I must repeat all those steps to put the replacement ECU in again and I have to wait until the engine is cold again to confirm that the problem is again fixed.

I understand why you are asking, and if I could do this when it's warm and comfy I'm sure I would. But once it's warm out, the car will run fine with either ECU. So the more I think about it, the less I think I want to go through it.

One could imagine it was a poor contact on one ECU pin or something, but those connectors are very large and robust and I don't see any corrosion or anything, so I'm convinced that it really is a difference inside the ECU. At a minimum, one can say that the problem is ECU related since the only change I made when the problem disappeared. Unless you want to theorize that disconnecting and reconnecting the battery fixed it. I think only an attorney could love that theory.

If someone else really wants to experiment with this, I'll send them the "bad" ECU.