@MattyRob
I FTP udated uninstall.php, even if appeared to be exactly the same as the one You suggested.
I then "deactivate"d subscribe2 from every site and finally "delete"d it from the dashboard.
I installed again Subscribe2 (v8.8) from the dashboard (automatic download and install).
I activated Subscribe2 in both sites, "italian" and "english"
I made a reset of both.
It DOES'NT work, it keeps resetting itself messing up the settings.
I confirm that it's still missing to enumerate the pages in the "english" site in the "Set default Subscribe2 page to:" setting. I have some pages but none of them is listed, just a "sample page" setting is there but I have no pages with such a name.
While the "send email from" setting is correctly set to both sites. I did not have to change it, this makes me suspect that none of the previous options was deleted from the sql database during uninstall.