I’m guessing that with Snow and Charming’s blessings, Regina has recast another version of the dark curse. The loophole being she has already sacrificed the heart of the thing she loved most. Or, possibly, in the missing year Robin and Regina fell in love, and her sacrifice was the memory of that love.
An actual heart isn’t necessary for Regina,as we saw when she cursed them back to EF, and sacrificed her son’s memory of her.
I suppose the drawback would be why did Zelena come with them. Unless, she didn’t come with them, but got there, somehow, on her own. Maybe she stole the Jolly Rodger, and it’s still there, docked in Storybrook, under another invisible cloak spell.
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