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.
Well, a triggered girl story might revolve around a girl being triggered by social situations. Say she has social anxiety and going to a large party or a crowded place triggers her. She might start to feel extremely uncomfortable, her heart races, and she has a strong urge to leave. It could be a story that shows how she copes with these triggered feelings over time, perhaps learning relaxation techniques or slowly facing her fears.