There doesn't appear to be much new info and without speaking Japanese it's very hard to look into this case, beyond what's already out there in English. As such, there's not much else I can add at this point beyond speculation.
We all know the general theory of the killings, the perpetrator came in through the window, killed Rei first, then attacked Mikio etc. However I think the following theory is just as valid -
The killer is young and dressed like a skater to the point a non skater wouldn't notice the difference. He approaches the Miyazawa house, secluded in the middle of the park, and rings the doorbell. Mikio answers and the killer tells him a story in order to gain entry. Remember, he's very young, he's on his own, it's late at night and he's a skater looking kid in a skate park with your house being the closest to said park. None of this would scream danger. He could say he was lost, or his parents hadn't arrived to pick him up, he was Ill and needed a doctor, he was worried someone was following him etc. There are plenty of options. I see no reason why Mikio would not let him into the house in such a scenario. This also explains the reports by the grandmother next door that the Miyazawa doorbell sounded around 10:30pm. Once inside the house it's possible he stayed for a while before attacking (we will come back to this a bit later) it's also possible he attacked fairly soon after entry. Either way, Mikio heads for the stairs, perhaps to show the youth the phone to 'ring for help', perhaps he was just going to the toilet, perhaps he went to get something to drink for them both whilst they 'waited', it's hard to say. But in any event, the killer follows Mikio up the stairs and attacks him near the top. This explains the knife wounds on the thighs and buttocks and the fact the wounds were in an upward direction. If you were stood on the stairs behind someone the back of the thighs and the buttocks would be the main target area available to strike. The killer then proceeds to attack Yasuko and Niina. Perhaps they were asleep, perhaps Yasuko was woken by the person and witnessed/heard what had happened to Mikio. Again, it's impossible to say, but if you did know a killer was on the second floor landing and you were in the loft, there would be nowhere to run, perhaps pretending to be asleep was the only option. Either way, they are murdered. Once the killings have taken place he stays in order to stem the blood flow to his wound and clean himself up. You can't be wandering around covered in blood with a gaping wound, it's a bit obvious. So he stems the blood flow (using the frozen ice cream and sanitary pads) then changes his shirt and jacket, taking some clean ones from Mikio. It would also fit that he would want to clean his shoes. Ice white trainers that are dripping with blood is a bit of a giveaway and you don't want to leave a trail of evidence once you leave the house. This explains why there was no blood evidence of the killer leaving. The bathroom screen is cut, perhaps he is thinking of using this as an escape route, perhaps he just wants to make it look like a robbery gone wrong. Rifling through the draws but not taking anything, stealing money, but not all of it, would certainly fit with that. This brings us to Rei. Two possibilities in the above scenario. First - Rei sleeps through the attacks, the killer becomes aware he is in the house, but is perhaps unsure of what to do. The position of the bloodied footprints in the bedroom are at an angle where you would be stood looking at the bed from about the doorway. Does he leave Rei or does he kill him? Once cleaned up either Rei wakes or the killer decides not to leave anyone alive and strangles him. He chooses this method as he doesn't want to be covered in blood again just after cleaning and changing clothes from the first attacks. Second - this comes back to what I said before about being unsure how long the killer was inside before launching the attack. It's possible, but I think less likely than option one, that he was invited upstairs by Mikio and kills Rei first then turns to the others. As I say, I think the first scenario is more likely. The killer then leaves the property.
Now, obviously that is just pure speculation, theres a lot of 'perhaps' in that and I'm wary to try and avoid getting into CSI/murder mystery territory. I'm not saying this is what happened. It's just a scenario that could have. But it's an example of an alternative. I just wonder if some of the assumptions around the case are part of the reason it hasn't been solved.
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