it doesnt have to be in the heat of battle, it could be after, when Jaime or Tyrion is on a slab. this battle is just a warmup to the finale. i think the WWs get no further than Winterfell and the survivors all band together against Cersei at KL. they already hinted that if NK is killed the other WW will just collapse. this show is called Game of Thrones, there is still game left to play. not gonna be much story left if Dany or Jon die. they need their army too to get down and breach KL. or a certain girl with a kill list who knows the secret way in and out of KL finishes Cersei when she thinks victory is at hand.
Ok, while I've only briefly skimmed the rest of this thread from p1636 to the end and may have missed it, there doesnt seem to be a mention of a rather interesting theory which solves a fair number of problems, including the time factor of the idea of Arya faceless-manning herself into the undead army, which I've shamelessly stolen from Mashable to post here and pretend its all my own work. It is as follows-
There is a major battle at Winterfell. Not that many major characters die. The good guys win. The undead army is devastated. There is much rejoicing, but Bran is uneasy because the Night King does not seem to have appeared. And in the midsts of celebration Bronn arrives, bearing news...
The Night King surviveD. Because he wasnt AT Winterfell. He took a portion of his forces swollen from the march south, and bypassed Winterfell to go hit Kings Landing by surprise, leaving most of his original army to seige or attack Winterfell as a distraction.
He wants to erase the history of man. All the major places have to fall, including Kings Landing and the Citadel. His army crossed the Wall with 100k or so. Danaerys has 100k dothraki alone, plus the unsullied and all the other factions troops, plus 2 dragons, all prepared behind stone walls they cant just pull down like they did the cheap wooden palisade, and loaded up with dragonglass. Thats a massive and very risky fight. Even more-so since the defenders will be targeting the Walkers. As we've seen, the walkers have their own groups of wights- targeting the walkers at Winterfell will pay handsome dividends for weakening that army and bringing victory regardless of the NK. Why attack Winterfell now, if instead you can go hit Kings Landing and maybe the Citadel too, and then come back for Winterfell after? Kings Landing is a city of a million people we're told, and no idea about the Citadel and nearby cities. He could come back with a million more wights, and thats a fight he could win, even against a prepared army. That wraps up Cersei in a surprise twist, explains previous seasons visions of a burned-out throneroom in KL, and why theres only 1 dragon shadow passing over KL in the visions and not 2, and perhaps extra significance to the books Sam removed from the Citadel which may be the only ones to survive. That gives Arya time to develop a wight face for use against the NK when that confrontation finally happens. It also means Bronn is on the road between KL and Winterfell in position to either see the NK fly over on his dragon or see the attack on KL and report it to Winterfell. If youre a writer you could have a buttload of fun having the Winterfell battle, having the survivors celebrate their victory, yay yay we get to live, and then Bronn arrives with the news that theyre now fucked x10.
So, could be complete balls of course, but its a good twist, it explains the visions, it explains Arya's recent actions, it explains why the NK hasnt been in any of the promos, it gives the issue of Danaerys and Aegon time to resolve, etc. It even gives time for the Red Priestesses to make themselves useful one (more?) time.
Here's a link to the bastards wot plagiarised me shamelessly!
Why the Night King won't be at the Battle of Winterfell on 'Game of Thrones'