One question to think about:
Let’s say that Emma’s plan to split her heart to save Hook works. Hook still needs replacing. Who is going to replace Hook? For every soul that is brought back to the land(s) of the living from the Underworld (or saved from it’s fate of entering the Underworld), another person must die. Will Rumple sacrifice himself , taking all of the Dark One’s curse with him?
I think the logic here is that the cost of the Curse was killing the thing you love most. The loophole was, you could pay the cost then split a heart to revive the price. Well, Hook’s already paid the price–he’s dead. Emma wants to enact the loophole by bringing him back with half of her heart.
Why that requires going to the Underworld is the confusing part. And yes, you get into the issue of soul math which is different from price math.
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing