The idea of Q saving humanity from the Borg seems particularly wrong. The whole basis of Q prematurely introducing humanity to the Borg was to give them a kick in the behind regarding their readiness to face such threats. There'd have been no point to the exercise if Q planned to protect them from the Borg anyway.
As for Shinzon...technically he was
a human, and a clone of Picard no less...had he succeeded, Q may have simply gained a new favorite human. Besides, Q had too much class to appear in such a crappy film.