If Tanjiro becomes a demon in fanfiction, he might lose his sense of humanity. His relationships with his friends and family would be severely strained. For example, his sister Nezuko, who is also a demon, might be affected in unexpected ways. He could struggle with his newfound bloodlust and the need to consume humans, which goes against his original kind - hearted nature.
Tanjiro's character development would take a dark turn. He would start from being a pure - hearted demon slayer to a conflicted being. His internal struggle between his human memories and his new demonic desires would be the main focus. For instance, he might remember his promise to his family but be unable to resist the urge to harm others.
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.
Tanjiro might use his sense of smell to detect hidden threats. Since his sense of smell is super - enhanced, he could sniff out any demons or traps in the way of saving Rengoku. For example, if there are demons disguised as allies, Tanjiro would be able to tell by their scent.