You know what I don’t understand is that Henry does love Regina so casting a spell to make someone love you who already does just seems like overkill.
Perhaps it isn’t Regina who is looking to cast this curse. Rumple just happens to find a crate with two potions missing that just happen to be the two needed to cast this empty hearted curse? Perhaps Rumple’s refusal to help is because he is trying to send the Charmings after Regina to distract them from what he is actually doing.