>"Ran Yakumo says with your cooperation, she believes I can be sent back in time to slightly before I was ever pulled into the future, and prevent the explosion in the Scarlet Devil Mansion's library that day. Or at the least, prevent Keine from being there. Her health deteriorated greatly after being crushed under a bookcase during the fire... and then the immortal Mokou went rampaging as a result of Keine's premature death and was permanently "dealt" with, which began Kaguya's downspiral. And that, I believe, probably contributed to her lover also leaving her later."
>Well, there was also possibly the matter of Reisen, but that's a less simple matter we can worry about latter.