Inosuke doesn't become a Hashira in the manga. The story progresses with him having other significant experiences and achievements instead of achieving that specific position.
It's hard to give an exact number. Different story arcs and battles led to the deaths of multiple hashira. Some died heroically, while others met their fate in unexpected ways. The count changes as the manga progresses.
Definitely not. Genya remains outside the ranks of Hashira throughout the manga. This is because of a combination of factors such as his skills, experiences, and the overall direction of the story.
There are many great 'watch and react kuroko fanfiction' works. You can start by looking on platforms like Archive of Our Own. Some popular ones often involve the unique relationships between the characters in the Kuroko series and how they develop in the fan - created scenarios.