Ahh... cool. Makes sense now. Althou, I suspect that the only thing that COULD be done is erase stuff. After all, aren't the pencils soul-bonded or some such?
Unless there are multiple sets of those pencils? And they can interact with each other? In terms of what has been sketched.
EDIT - would Amy show up at all? Given that she is sort of a background character with some magic and such?
Also, an interesting possibility is that the Mayor turning into a demon isn't a -bad- thing per say. He could still be Richard Wilkins, minus the soul, in a giant snake body. All nice and polite and wanting to TAKE OVER THE WORLD as opposed to destroying it.