In the original book's 3rd chapter, which is the arc where Yuzuki and the protagonist go to visit the old Katabuchi family estate, they never visit the house in person and the entire conversation takes place in a conference room. This was likely changed to break up a bit of the monotony that constant conversations with no actions would have in the format of a manga, but in an effort to make the manga even more interesting, Keita appears for some reason. In the original book, the only moment the reader learns more about Keita is in the letter that claims to have been sent by him, so having him actively interact with Yuzuki and the protagonist is a change unique to the manga. Despite this seeming like an interesting change on the surface, this one change ruins the final twist of Strange Houses irreparably.
You may be wondering, "why is it such a bad thing to have Keita appear at this point in the story?", and that all has to do with the final afterword in the book. For context, while Strange Houses is itself a fictional story, it also exists as a novel within the author Uketsu's metanarrative, where Kurihara is a character who also plays a role. In the afterword written by Kurihara, he complains that in the book, Uketsu did not actively include all the information shared by him in the final novel, and that there were details left out in order not to cause trouble. Kurihara then explains the following from his point of view:
- There isn't actually any proof stating that Keita is the one who wrote the letter that Yoshie read, since after all, it's been established that Yoshie has been dishonest about her involvement in the plot of the book. Therefore, we can't actually conclude with relative certainty Keita was the author of the letter, so facts that can't be confirmed by other characters shouldn't be trusted as being the full truth.
- If the letter is to be taken as containing some fact, though, there are a few concerning implied facts. For starters, it's stated that the entire Katabuchi family had control over the design of the Saitama and Tokyo houses, which would include Ayano and Keita. Therefore, it's reasonable to assume that any design choices made in the houses could have included their input.
- There also wouldn't actually be any reason for the husband to keep watch over the living room as originally theorized, since there shouldn't have actually ever been anyone killed, and thus, no one seeking revenge. Therefore, the strange bedroom on the first floor with a window could potentially serve another purpose: instead of serving as a way for Keita to watch over others, perhaps it served as a way for him to be watched by Ayano.
- You know, they never identified the missing body in the first chapter. Strange for it to turn up right after Keita went missing...
To put Kurihara's conclusions together (he keeps it rather vague in the original text, but there's an obvious conclusion), Kurihara believes that the story of Ayano and Keita was likely more dark than the book's writing portrays, and Ayano was much more involved than she acts. He also seems to believe that the body found near the start of the story was likely Keita, and that Keita was possibly murdered by Ayano in the house.
... However, if you've not already put it together, if you have Keita alive and walking during the Katabuchi family estate segment, you can't exactly have him be dead in the first segment of the book, and so, the manga adaptation quite literally outright ignored this entire plot thread in favor for a fine but underwhelming, sweet ending. I likely explained the reasoning poorly, as I'm typing this solely from my memory of the book, but I hope this gives you a good idea on why the ending might feel as underwhelming as it does. You didn't get all the pieces of it!!
