paleaswater remarked on the hard to get-ness of stories 3&4 in vol 19, so I reread. Umm, Bottom of the Stone Stairs seems fairly straightforward except that the action shifts into kekkai-land a lot and Ritsu is a bit more helpful than he usually is. The fourth story OTOH was one I'd dismissed as 'OK whatever' until the reread.
Finally realize that what happened is this: daughter's hair all fell out through stress at work. Wigs being expensive, Mother gave her one that had belonged to either her own mother or her grandmother. (Pesky Japanese habit of referring to people from the POV of the person they're talking to. Is why Kinu refers to her own father as Grandfather, and Aoarashi as Father, when talking to Ritsu. In this case, given relative ages and almost real time time development, I think it must be mother's mother.)
Wig retains the personality and obsessions of Grandma, and possesses daughter. This is why she thinks she's a daughter-in-law of the family, widow of the oldest son who died in South-east Asia. (Malaysia, I assume, during the war, obviously: can't identify where because the place name is in katakana-good-luck. Inpaaru?) Is also, I realize on third read, why she thinks Ritsu's name is Ryou when they first meet. Is why she knows there's valuables hidden in the wall and comes prepared to get them back.
Meanwhile Mom has medical condition I'm not sure of (mild stroke, I think) mimicking first stages of Alzheimer's. I suppose this *might* explain why she thinks a total stranger is her son. That part I find a bit hard to believe, but no matter. Imposter son is the one who insists he's stationed in Okinawa, which may account for the 'is he in Singapore or is he in Okinawa or is he dead?' confusion.
Does that help any?