We saw in 1×02 that the curse has to have a price, yet Snow and Hook conveniently found a loophole for that – contradiction.
Actually it is still canon for paying the price for the curse. Even though Snow shared her heart
her husband, she still paid the price by crushing his former heart.
Hook does not count, as the dark one, the darkness is able to form into Nimue, so that is where Nimue literally paid the price by crushing Merlin’s heart (with Hook’s help). Let’s just say the dark ones are gifted.
We are thought that dead is dead no matter what, yet everyone goes to the underworld to save Hook – contradiction
There will always be a price to try and resurrect the dead. Like with a life exchange. And it is revealed that there will be a consequence for Snowing for leaving their son. It may have something to do with the shared heart while they are in the underworld. And Emma would never allow that. The shared heart thing would obviously not work for Killian. That is why there is unfinished business and emotional closures in this arc.