The mindwipe's not a possibility. Magic isn't real. It's escapist fantasy. The amulet didn't do anything to him, it was swamp gas, because if the amulet did do anything to him, that would be magic, and magic isn't real. He doesn't have to worry about mindwipes, because if Zatanna's backwards words could change reality like that, that would be magic, and magic isn't real. If it were, after all, the amulet would be magic, and there'd be no need for some elaborate explanation about swamp gas hallucinations.