Cursed (emphasis mine): You cannot use monster actions or skills denoted with a :arrow: At the start of your turn, test :knowledge:. If you pass, discard this card or token. If you fail, keep this card or token.
Stunned (emphasis mine): :arrow:: Discard this card or token. This is the only action you may perform on your turn while you have this card or token.
Cursed does NOT prevent you from becoming un-stunned. It only prevents you from using SKILLS that have an arrow, not any action (which is what an arrow denotes). As such, even if you failed, you could still remove Stunned, could still move, attack, etc.
As for Syndrael's feat, since the move action is NOT on your turn, she could use it to give a stunned hero a move action. Note that stunned isn't intended to prevent heroes from doing anything. They can still spend fatigue to move and use skills that don't require actions. Being that Stunned and Syndrael's feat were part of the same expansion, I believe the "on your turn" was specifically added to allow for such a case.