Timeline-wise, this would take place between the 1st and 2nd chapters, right?
Humanity starts a nuclear war
Large portions of the world's surface are contaminated by nuclear radiation
Humanity starts building a layered city to escape the radiation on the surface
The city grows talled and taller
To be specific, Kyara is the highest grade of Aloeswood in Japan. It's also the namesake of the "Strongest Referee Number Zero" in the gambling manga "Usogui".
"Hornfels" and "Feldspar" also happen to be names of characters in the space exploration game Outer Wilds. All Hearthians in the game are named after rocks. Maybe the mangaka put in a reference to it?
I was re-reading the hangman arc and noticed that Yakou set a "1-minute time limit" there too. Is that something he does habitually? Did Usogui pick up on that? Is that why he wanted Yakou specifically to be the one choosing the game and rules?
Earlier, Youmu had 666789bam, called pon on the 6bam, and discarded 9bam, switching from a closed yaku-less hand to an open tanyao. With Remilia's skill, and apparent ability to see who is fated to win, she could definitely deduce that Youmu had the fourth 6bam all along.
Also, the 6bam wait...
Daigo's hand would be haneman and bust Majin out, leaving him in third place. In fact, with that hand even any tsumo would bust Majin out, so he can only ron Kiriya or Maguro.
What's with the final scores? Nishikata should have about 25000 more points and Hiroyuki should have about 25000 less points. That would move Nishikata to second place, which would give the East an extra 20 points in the total score. It also seems like there has been an Oka bonus - basically +10...