eventually I could find where the problem is.. just a little bug, enough to virtually disable this feature though.
Inside "TrackDirectorAction.inc.php" - notifyReviewer function - at line 230 there was:
$reviewerAccessKeysEnabled = $conference->getSetting('reviewerAccessKeysEnabled');
where instead it should be:
$reviewerAccessKeysEnabled = $schedConf->getSetting('reviewerAccessKeysEnabled');
In other words, that line was telling the system to find a setting belonging to the general conference, whereas it actually belongs only to the specific scheduled one.
This meant that the real setting, in that context, was basically ignored and so the one-click feature could not work.
Same problem with remindReviewer, some lines below.
Now it works fine ...
Hope this could help other guys and OCS developer as well, of course.