So is Yotsuha's watch powered by her lifespan and when she runs out she dies? Or she made a deal with the devil that she dies when Uichi is happy (ironically making him unhappy?)
There's a second person time traveling to make things worse? That kills the entire story. Like, fuck off. No. That's a terrible idea and torpedoes the whole thing. This doesn't need to be a 100+ chapter story