[ETA - I figured I'd add this: absolutely don't click my spoiler tag unless you don't plan to read this. I don't actually give away who did it, but there are spoilers in my wording.]
Finished! I can now say that, no matter how long I sat down and thought about the clues, I'd never have figured everything out. As it was, I managed to correctly guess 1.5 things, sort of:
that the Kazue the police officer met was not the real Kazue (that one was pretty obvious, even though I couldn't figure out the reason for it) and that the murderer was one of the murdered young women.
I didn't pick the right young woman or figure out any of the other key points of the murder, and I thought she'd actually been killed along with the other five women. My best theory was that Tomoko and Heitaro worked together to commit the murders, and then Heitaro betrayed Tomoko and killed her, leaving the body where he was because, I don't know, after burying five other people with Tomoko's help, he was too tired to bury her on his own or something. A theory which had lots of holes and didn't account for the problem of the almost certainly fake Kazue the police officer met. Unless his letter was a fabrication, but then why mention that the Kazue he met didn't quite look like the photo of Kazue in the newspaper? See, problems.
The actual explanation made sense, although it relied a lot on luck and the police either missing key details or being unable to get key information. And dang did the killer have to be ice cold.
I wish the second book in the series, The Locked Room of Pythagoras, were available in English.
Oh, and I'm going to be counting this for my Locked Room Mystery square. I haven't done a Bingo update in ages - I need to do that when I get home.