I wonder if the Dorothy scene which she is in the blue dress is a flashback with the melting and Zelena figured out how to over come that. Now it just requires removing the necklace to rid her powers and then using the “ripping out the heart” method to kill her or to just excile her someplace.
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Nothing is impossible on OUAT. Everything has a way of repeating itself. Keeper of the Keeper tread: https://oncepodcast.com/forums/topic/official-keeper-thread-3/#post-217924