I don’t necessarily think this curse is exactly the same as the one Regina cast the first time – that was the “Dark Curse” and it was made to destroy everyone’s happy ending but to give Regina her own Happy Ending. For that reason she had to sacrifice the heart of the thing she loved most.
This time around I think they just figured out the only way to get back to Emma was to curse everyone again by sending them back to SB just like before but missing only their memories of the past year – that isn’t the Dark Curse. In that case I don’t see why you would need an actual human heart to cast the curse but if you had to sacrifice the THING you loved most Hook would be perfect because of the Jolly Roger. Maybe it’s just some kind of major sacrifice that is required.
As far as Zelena remembering the only thing I can come up with is that since she is so powerful the curse doesn’t affect her or since she wasn’t affected by the original curse she isn’t affected by this one either.
I am not sure why Hook wouldn’t tell everyone what is going on but there has to be something else that went down during the missing year that Hook doesn’t want Emma to know about. I am having difficulty believing that it is just guilt over the thing with Ariel and our theory that he in some way sacrificed the JR for her that is making him want to keep it from her.
My theories are typically wrong and I expect this one will probably be too!