First, Sebastian Stan is finished filming Captain America: Winter Soldier and has nothing scheduled for another film (From IMDB). Wouldn’t he want to spend some time with his girlfriend, Jennifer Morrison. This is what I would do.
Whatever he’s up to that would prevent him being on the show, I dunno the specifics of, but Jane Espenson did say this the other day at DragonCon:
They want Sebastian Stan back. Miss Espenson said he was probably the only actor in history to get his own spin off and not be able to be in it. She did say that if they can get him back this season, it would be late in the season because of scheduling.
http://onceuponanobsession.tumblr.com/post/60185465175/i-learned-season-three-spoilers-at-dragon-con
The reason why Jefferson couldn’t make the hat magical is because there was no magic in Storybrooke.
Jefferson couldn’t make a magical hat because he doesn’t have the ability to make a magical hat. That’s what caused him to become the Mad Hatter, when he was trapped in WL, a world full of magic, locked away by QoH until he could make a magical hat. And he made a frillion hats, but none were magical,