Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
Nope. Ryomen Sukuna has no basis in reality. It's a product of the creative minds of the writers and illustrators who designed a captivating fictional character for the enjoyment of the audience.
Well, in such fanfictions, a common theme is the struggle against nature. The cold is a powerful adversary, and Naruto has to use his ninja skills and willpower to overcome it. There could also be themes related to self - discovery. Facing the threat of freezing to death might make Naruto realize some things about himself, like his own mortality or how much he values his life and the lives of others. And sometimes, there could be a theme of sacrifice, where Naruto might put himself in even more danger to protect someone else in the freezing situation.
In the manga, Sukuna doesn't possess all 20 fingers. There are specific story arcs and events where his finger count changes based on the plot developments.
In the manga, Sukuna does acquire all his fingers over the course of the story. This is a significant plot development that has major implications for the storyline and character dynamics.