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.
The Soul Reapers would be very confused at first. They are used to Aizen being a certain way, and Naruto's different personality would be a shock. Ichigo might be curious and see Naruto - Aizen as an ally against the common enemies. Orihime could be one of the first to trust Naruto - Aizen because of his kind - hearted nature that she would sense. And the Espadas would probably be on high alert, not knowing what to expect from this new Aizen.
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.
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.