Maybe it’s not that the half a heart wouldn’t have saved Hook, but it would have hurt Emma. (That’s what started to happen.)
So why is that Emma can’t live with a whole heart but Snow can? I mean the only conclusion I think I’d draw from that is that Hook and Emma aren’t as true love as Snow White (because apparently true love can be randomly quantified by some heretofore unexplained set of measurements???) If anyone could live with half a heart you’d think it’d be True Love Incarnate.
So if Pan had been given Robin’s heart, it would have killed Robin. Which fits the whole “life for a life” thing.
Maybe but it’s still a physical heart inside a nonphysical essence. I don’t know how that works at all let alone how Pan was going to be able to interact with the topside Real World.