Giyu survives in the manga. The plot doesn't lead to his death. His character has many adventures and developments that keep him alive and active in 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.
Their relationship can be developed through shared experiences. For example, if they both get injured in a battle and have to rely on each other for survival while waiting for help. This would force them to open up and communicate more, breaking down Giyu's usual stoic exterior and allowing Shinobu to see a more vulnerable side of him.
A plot could revolve around a mystery illness that is affecting the Demon Slayer Corps. Shinobu, being the expert on poisons and medicine, tries to find a cure. Giyu gets involved when he discovers that some of his comrades are showing symptoms. Together, they suspect that a new type of demon or a hidden enemy within the corps is causing this. They have to investigate, and during this process, they face many challenges and also grow closer as they share their concerns and ideas.
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.