If Shou decided to send Yu back, confident he could change the future, why did he suggest everyone else stay to rebuild a future that, if his confidence was well-founded, wouldn't need rebuilt? If everyone had gone, they could have not only lived in the non-apocalyptic future, they could have spent the years leading up to it with their families, abundant everything, and no monsters, mutants or poisonous fogs. Maybe they could have even helped ensure the future would go well. Shou was a good leader for a kid, but unless I'm missing something huge, he really tripped at the finish line.
Also, why was Shou/Sakiko/Otomo never addressed after Otomo's confession? Does she still love Shou? Did he tell her who he liked more? Did Otomo win her heart with one speech? Are we supposed to just guess what happened? Are they pretending nothing happened?
Apparently, Yu did manage to change the future, but into what? Everyone looks happy, but still messed up. Didn't they change with the timeline? Was the theory that the dead would be alive proven false? Or did he not change it but everyone is okay with it?