The Hashiras would likely show concern. Some, like Shinobu, would immediately start looking into possible medical solutions. Giyuu might be more reserved in his worry but would still keep a close eye on Tanjiro. The other demon slayers would also be affected. They look up to Tanjiro, so his sickness would lower the morale of the group. They would try to support each other while waiting for Tanjiro to recover.
An emotionless girl reader may seem indifferent to the themes in an Underswap fanfiction at first. But the unique themes that often come with the Underswap concept, such as role - reversals and alternate universes, could start to make her think more deeply. She might not feel excited or intrigued in the traditional sense, but she could start to engage with the themes on a cognitive level. This could lead to a sort of internal exploration where she begins to compare the themes in the fanfiction to her own experiences (even if she doesn't have strong emotional ones). And who knows, this might be the start of her developing some form of emotional response in the long run.
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.