Yup, sorry for hijacking the thread.
As said, rules seem to be pretty clear on how long a spell cast remains in effect, so that Zebulon seems to allow recasting of the same spell in case the usual conditions are met.
If you cast it and fail, you can't use that hand for that round of combat, but that's it, you can refresh & recast and use it with your other hand. Point is that using a spell "blocks" you one (or two hands), not having the card exhausted.
If you're still bothered by this, just play it the way you feel more comfortable with. Or use Ockham's razor and ask FFG, Tim usually is rather quick in covering Arkham questions.