I mentioned this in another post, but it applies here too. There is a lot of Wonderland clues, but there are more characters in Wonderland than the White Rabbit. (Though March Hare has been suggested as well). What if he’s Cheshire Cat??? Ches can disappear and reappear at will, perhaps this is a way he avoided the curse? Or what if he’s Tweedledum and the postcard came from Tweedledee (visa versa) that is in Storybrooke, we just haven’t met him yet? Just some more Wonderland thoughts for ya!