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.
Some fans would be really shocked and sad. Kakashi is a very popular character, and his death at the hands of Obito would be a huge blow to many. They might feel angry towards the author for creating such a tragic scenario.
Sakura would probably be very shocked at first. She might be worried about Naruto's well - being and also a bit fearful of the combined entity. Kakashi, on the other hand, would be more understanding. Given his connection to both Naruto and Obito, he would try to communicate with the merged form to figure out what happened. Sasuke would be initially suspicious, thinking it could be some new threat. But after seeing that Naruto's essence is still there, he might be more accepting and see it as a new power to use against their common enemies.
Sakura may also use her medical ninjutsu in a creative way. For instance, she could try to numb the part of Obito's body that is restraining her by using her precise chakra control. Or she might use some kind of medical jutsu to create a distraction, like a bright flash of light or a small explosion of medical energy.
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.