It's hard to give an exact number, but I'd say Itadori has eaten at least 10 Sukuna fingers in the manga. The number keeps fluctuating based on the plot and his encounters with various challenges.
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.
Perhaps it was because he did not pay attention to the story and cohesion in the writing process, resulting in the plot and character descriptions not being clear enough and lacking in expressiveness and emotional resonance. Writing was a creative process. Everyone's style and characteristics were different. You can improve your writing skills through repeated revision and practice to better express the story and characters.
Well, one reason could be genetics. In the Naruto universe, Kushina was Naruto's mother. So in fanfiction, the author might want to emphasize this genetic connection by making Naruto look like her. It could also be for a plot device. Maybe there's a story where Naruto's resemblance to Kushina plays an important role in uncovering some long - lost family secrets or in a prophecy.
Often in fanfiction, the resemblance between Naruto and Minato can be for emotional impact. When other characters see Naruto looking like Minato, it can stir up memories and emotions related to Minato. This can create interesting character interactions, like when the older generation sees Naruto and is reminded of Minato. It also adds an extra layer to Naruto's character as he might start to understand more about his father through this physical resemblance, like how people might react to him differently because of it.