Greyjoys will own everyone in the end. It is known.
1. Writer never described how dead beeing brougt back to live beyond the wall.
Others' magic. Martin also never described why the seasons in this world are of random length. When people ask him what is the reason for that, he says "Magic".
2. Beric Dondarrion - every majore character in the book seem to be ignoring the part where this guy was resurrected multiplied times. No one is that ignorant.
His followers in BwB know that he was resurrected multiple times. Most of the people in Westeros do not believe in magic or believe that it died with the dragons.
There's a scene in ACOK I think, where Alliser Thorne arrives at court in King's Landing, presents the rotted hand of the wight and says that the dead rise beyond the Wall. No one, including Tyrion, believes him and the joke advice he gets from Tyrion is "Bury your dead deeper so they won't rise".