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.
There are fanfictions where Naruto uses the black fire after making a pact with a powerful entity. This entity could have granted him the ability to use the black fire. He may use it in a strategic way during battles, like surrounding his opponents with walls of black fire to trap them.
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.