If the grandfather is very warm and accepting, Shikaneji would feel relieved and happy. They might open up more about their relationship and their dreams. Nejire could start chatting excitedly about their future together, and Shinsou would relax and join in the conversation more. However, if the grandfather is not so friendly, they would be disappointed and perhaps a bit defensive.
Muzan was ultimately defeated by the collective efforts of Tanjiro, Nezuko, and the other demon slayers. Their teamwork and perseverance were key to ending Muzan's reign of terror in the manga.
Well, muzan x reader fanfiction often explores a very interesting relationship. In such fanfictions, the writer usually dives deep into the dynamic between muzan, who is a complex and often dark character, and the reader - insert, which can be a very exciting exploration of power, mystery, and sometimes a touch of romance. It allows fans to put themselves in a fictional world with muzan and experience different scenarios.