Tedd's super special magic x-factor mutant whatever the whatsit ability to see magic tells her a LOT, but there can still be a lot to keep track of and not all spells are terribly straightforward.
(That's writer speak for "ha ha, I can make Tedd be however sure I need him to be about how a given spell works")
Which makes sense anyway, SO THERE. I've probably made the analogy before, but Tedd is able to see a sort of blueprint for how a spell works. Some can be looked at and make sense at a glance. Others require a closer look, or might just be baffling altogether.
And yes, this does mean that it would probably be over Tedd's head to give everyone a way to cast the fairy doll spell at this point.