So here is my guess on how endings would go, two guesses, the manga is called 100 Ghost Stories, and 100 stories must be told:
1) In chapter 100, Yuuma's mom feels sorry for her son so she is the one telling the last story, and somehow interrupts the ritual, it fails, and Hina gets her body back while Yuuma's mom's soul leaves the mortal realm for good.
2) The ending is in chapter 101, after telling 100 stories equal to 100 chapters. It's a plot twist as the manga ends at chapter 101 instead of chapter 100.