Earlier, I wrote, “Anastasia Griffith is listed in the credits of episode 18 as Kathryn/Abigail. Perhaps Kathryn's not dead after all. The heart must belong to someone else.” I'd like to amend that theory. Either it's not Kathryn's heart, or it is her magical heart that Regina removed in the same manner we saw Regina remove Graham's heart in episode 7. Kathryn could still be alive but in the hospital basement or some other holding cell, and Regina planted Kathryn's magical heart in a box with Mary Margaret's fingerprints.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy