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.
Au999.9 was pure gold, and the gold content was no less than 999.9%. It was the highest value in the naming of jewelry. It was dark yellow in color and extremely pure. It shone with golden light and was bright. The markings were usually printed with words such as "Thousand Gold,""999 Gold," or "G999."