I am not getting too worked up about the Night King's plan or his magic since magic kind of messes up plots anyway. Melisandre was pretty important in that last battle, which means her magic is pretty good despite the fact that she is a total dipscalito that has stumbled from screwup to screwup throughout the entire series.
As it relates to the Night King, if you think about the complexity of managing an army of 100,000 mindless wights, who speak multiple languages and are even multiple species, I am not sure why his network didn't just crash.
I think Bronn made a comment in Season 2 before the Battle of the Blackwaterr that he thought plenty of the Gold Cloaks were willing to kill for the King, but he didn't think too many of them were willing to die for the King. We may see that at some point with the Golden Company.
I think the Hound and Arya are going to go after the Mountain together, and it won't be pretty for any of them. I think it will be a scene that will make the brutality of Brienne/Hound or Tormund/Umber look like ballet.