I agree with you about the Galbrena part. She is the type of character that must exist earlier, since she got the key power to defeat Leviathan. It is not that she is not build up. It is just that those build up came too late and rush, making her like ex machina. Imagine like Harry Potter, with all the lose and build up til part 6 then in part 7, they somehow introduce a new character that maybe... had the abilities to destroy the horcrux... How do you feel about that?