Regina has, we can assume, quite a collection of hearts. Dead hearts. She was about to add the one the Huntsman brought her thinking he’d taken it from Snow non-magically after killing her. One from her collection is a small price to pay to frame MM (and she may get the heart back in the end anyway). Regina can conjure false phone records, so intercepting and altering lab test results shouldn’t be too difficult, “proving” that this heart belongs to Kathryn. I still think Kathryn is being held somewhere alive.
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