Greg responds...

When Puck transforms into Owen, he transforms into a human being, with all a human's weaknesses and vulnerabilities. Now Owen can transform back into Puck, but he can't perform magic as Owen. And Demona caught Owen off- guard with a spell that affected his all-too-human form. I suppose Owen could have transformed back into Puck in between the two nights of the curse, but that would mean he couldn't help Xanatos at all. Remember their deal: one wish from the Puck or a lifetime of service from Owen. Xanatos chose Owen.

As for "The Mirror", Xanatos went without Owen for a few hours. Who knows whether he even noticed.