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I checked some stills and plot summaries of Kihachirō Kawamoto's shorts online, but none of them were what I was looking for. Of course it's still possible it's some film by him I didn't come across... Though I seem to recall the short was made in Europe or USA, not in Japan, despite the Japanese subject matter. But thanks for the tip anyway!
Edited by TuomasYou can also try the Big Cartoon Database Forum - Mystery Cartoons.
http://forum.bcdb.com/forum/Questions_About_Toons_C17/Mystery_Cartoon_F15/
This one has a different story, but meets the "technical" description - puppet animation of a stage play, a narrator, scenes change by quickly shifting around props, and a mood whiplash at the end:
https://www.youtube.com/watch?v=t0d1EsHJH9Q
Maybe this is a starting point

What I'm looking for is a stop-motion animated short, made in the early or mid-90s. It tells a sort of a faux Japanese folk tale about a samurai who has to save a girl from a demon. The animation is the made to look as if we're watching a play, so the camera remains stationary, and instead of cutting from scene to another the props behind the characters change all the time. Also, since it's supposed to be a play, it has an on-stage narrator, an old guy with a white beard. I don't remember much about the story, except that the samurai beats the demon and gets the girl. The narrator then walks to the stage, takes of a mask he was wearing and reveals that he's not actually an old guy but the samurai of the story, who's then joined by the girl he saved. You'd think the story would end there, but all of a sudden the camera moves (for the first time in the film) much closer to the characters than previously, and we see the demon attack the samurai and the girl. The demon kills both the samurai and the girl in a rather gory manner, and the short ends with that. This is quite a Mood Whiplash, as the rest of the story was more like a classic children's fable. I think the short won some big price for short films, but I can't remember which one.
If it helps any, the short was made with stop-motion animated puppets, like Nightmare Before Christmas.
Edited by Tuomas