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.
One possible plot idea could be the exploration of how Mitsuri and Obanai deal with the impending arrival of their baby. Maybe they face challenges like preparing a safe home in their demon - slayer world. Another could be the reactions of their friends and comrades to the news.