This finishes the story of the main anime, its a direct continuation. The world building continues to be as interesting as before, although the heightened suspension of disbelief continues to stay necessary. I dont think this is a problem with the writing, as anime is a medium not unfamiliar with such over dramatization of what's physically possible. Without spoiling the plot, this movie will confirm your theories about the state of the world and it's likely purpose if you have been paying careful attention until now, there isnt some huge trigger esk plot twist coming here. Overall, decent show, greatly held back by its unusual look, although I mustnote that intentionally using the uncanny effect of bad liquid physics for the snow sea to make it terrifying to the audience is a great show of artistic ability.
Another aspect that makes it obvious the people that made this cared to get it right is how both the sociological and environmental aspects of the world building is generally very clever and well telegraphed if you take a step back and think about this world from the perspective of the audience, instead of the characters. The way the great snow chasm, something initially seeming utterly nonsensical, "works" is a great example of this. I do not wish to ruin the "aha!" moment for you, but feel free to DM me if you cant figure it out yourself