two things about Viserion
1: The three-headed dragon theory isn't necessarily done. It seems plausible that Jon, who we know represents ice, may end up riding the ice dragon.
2: I dislike the walkers using Viserion this way. Simply because it seems a little too deus ex machina. Like, did they ever really have any plan for getting over/around/through the Wall? Or did they just figure "march until something shows up"? Ie, what would they have done had they not gotten their ice dragon?