Counterfeit products is going to focus on Tanjiro and Michikatsu.
Michikatsu realizes he loves his brother too late. When his brother is already old and his body cut in half. He proceeds to have a mental break down. He wanders Japan as Yoriichi did in hopes of finding something to connect him to his dead brother. He follows Muzan's orders half hearted now.
Until one day when he is in a village by a mountain, he finds him. His brother. Michikatsu is not sure how Yoriichi is back but he is not going to look a gift horse in the mouth.
He follows Yoriichi home only to be taken aback by the people there. People who call his brother theirs. It enrages him to think that they have tricked Yoriichi.
He kills the Kamados and uses a blood technique (that I am giving him because *.✧angst*.✧) to alter Tanjiro's memories into thinking he is Yoriichi
Yoriichi remembers moving into the mountains with his brother to get away from their father. It had been peaceful. Until a demon attacked. Now the neighbors were dead and his brother had been turned into a demon.
Yoriichi pulls his brother close to him and promises to find a way to turn him back to human.