Yooo that was nasty with Muzan abd the brains and hearts. Just ew.
Tanjirou gets a glimpse of the final form, maybe thats all he needs to figure it out?? Seems like the flashback ends here. Too bad we didnt get to see how the brother got turned into a demon (was it Muzan too??? Like at the same time? Did he do it before he meet the younger brother?) Or maybe I'm misremembering.
I can feel Tamayo's frustration. He had it yet he ruined it because he wanted to ask his random question at the worst possible timing. You don't reason with monstruosities that kill people in mass, you cut them down and call it a day. Guess he saw the inmediate consecuences anyways so It all works out