Good brainwaves, 4wd, that can all be explored.
Anyway, Image Mirror' author, in the very same day I contacted him, implemented my suggestion: I tested it and replied (it was ok), and in the morning I had a mail announcing that on the website a new fresh release of the software was available. Fantastic.
From my tests, it seems to work very well (I still need to investigate on some problems, but I don't know if they're related to Image Mirror).
Unfortunately, there's no way to avoid the need of two screens, because Image Mirror can only flip a rectangular area that is actually shown somewhere in the desktop.
Making a software capable of just flipping the screen, in Windows, seems to be a very challenging task, if possible.
At the moment, I think Image Mirror is the best solution - of course let me know here if some better solution is found.