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.
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.
Yes, in some fanfictions, Tanjiro is depicted as the reincarnation of Yoriichi. This concept adds an interesting layer to the story as Yoriichi was an extremely powerful and legendary figure in the original lore. The idea allows for new storylines where Tanjiro might inherit some of Yoriichi's skills or have a deeper connection to the past events related to Yoriichi.
Tanjiro's fate in the manga is that he ultimately passed away. This event was a significant part of the story's development and had a profound impact on the other characters and the overall plot.