I have a feeling Sarah is not "the one". Not b/c she doesn't fit, or b/c he doesn't want her to be ... I just imagine him falling in love with her ... truly loving her, and then coming to realize she's not the one he was meant to find. Then, in the midst of it all, he finds the one ... but with his heart being with Sarah, he's faced with a choice of losing her just to obtain mortality.
I have the exact same feeling too, I'm afraid she's not the one, and that JA will find another possible "One" after dating the doc for some time, having to face a difficult choice... But, anyway, the spell said something about "wedding souls", whatever it means, I bet it's not as simple as a normal marriage, and of course, the doc still has to get the divorce, so we will have to wait for some weeks until this works out one way or another. And I don't think she fell too fast, come on, they are adults, as long as they take precautions, and if she's not with her husband any more, I don't see any trouble in that. Plus, I don't really think too many women would have resisted so much attention by a handsome and misterious guy, though I may be wrong, I'm not a woman and I don't like guys, after all.