Gonna put on my Kurihara specs and write down a theory I've been building the past few weeks.
I said above that I suspect granny is connected to every picture/murder in this story, and here's my thinking.
I still believe she's A-ko, and therefore murdered her mother, although another reader pointed out the psychologist looks around the same age as granny, which is giving me pause for thought. Makes me wonder if the psychologist is a makeup wizard, too.
As for Miura's murder, I believe the time of death was somewhat spoiled by chapter 9, it shows that he was still alive when it was dark out, so the supposed 5:00 pm time for the murder seems wrong. As another reader pointed out, his drawing seems to indicate he was alive when it was bright again, sometime early morning before he was discovered. Whatever the case, the time of death is now uncertain. Now Toyokawa and Miura's wife had alibis for early the next morning after the hike, so it seems unlikely they were the murderers (if they were, the murder would have to be around 3-4 am, too dark for Miura's drawing). But another reader had a theory that kind of appeals to me, which is that Yuki was an accomplice. Now I don't see any evidence or motive for this, but a few things do fall into place if this is true. Yuki and Miura's wife perhaps worked together to tie up Miura, the wife leaves the mountain, leaving Yuki to do the murdering, and so the wife has an alibi for the next morning (and Yuki seemingly doesn't).
As for Toyokawa, it seems suspicious that he disappeared from the trail the day of the hike, so it seems pretty plausible he was stalking Miura, too. Makes me wonder if maybe all three suspects were working together, and therefore all three of them now have dirt on each other. Toyokawa took advantage of this by making moves on the wife, she was unable to do much about it because he now had dirt on her. Makes me wonder what he's doing during the stalker incident, or if he's even still alive.
As for Yuki, I don't really know what motive she could possibly have to be a part of the murder. But the way she hangs around the surviving Miura family seems to show that Yuki and Miura's wife had some sort of prior relationship (again, please don't be an incest story...), or maybe she was forced into this situation in some way. Years later, Yuki marries Mr. Miura's son, who I strongly believe is Takeshi (and therefore Miura's wife is granny). For one thing (and another reader already pointed this out), granny's and Takeshi's ages more or less matches Miura's wife and son (actually, the son MIGHT look a bit too young...). If my estimation's correct, granny would be 31-41 at the time of Miura's murder, and Takeshi would be 11-12. Another reason is, in his blog, Takeshi implied he wasn't allowed to play freely when he was little, and perhaps had some sort of fear of father figures, which does match what was revealed in chapter 9 during the interview with Miura's wife.
Perhaps Yuki felt guilt over being an accomplice, or maybe she and granny had a falling out, and granny feared that Yuki would rat her out. To silence her, granny planned to murder Yuki. Yuki, sensing that something was happening, made the future prediction drawings to tell Takeshi what was happening, but couldn't directly tell him what happened, because if she did, she would reveal that she was an accomplice (otherwise how would she know granny's the murderer of his father?). Now I had a hard time understanding how granny could have killed her during childbirth, but after reading the blog a few times, I wonder if it has something to do with the vitamin supplements. Yuki's heart palpitations seemed to have started after she started taking supplements. Since granny was the "third person" living with Takeshi and Yuki (or so I believe), maybe granny replaced or added something into the supplements and was counting on the strain of childbirth to give her some sort of heart attack. I only think this because Yuki's heart problem to me is a pretty conspicuous detail in the blog.
Meanwhile, Kumai survived his cancer (it's been 20 years, so presumably he got treatment), but maybe it came back and, figuring he had nothing to lose as a dying man, started to stalk granny to go after the one case he couldn't solve - instead, gets stabbed.
That's my current theory, but there are some holes. There's not much evidence to some of the stuff I said, I don't see any motive for why Yuki would be involved in her crush's murder (like I said before, perhaps she was forced to do so?), and inducing a heart attack to kill someone during childbirth seems to be a shot in the dark at best. Plus, unlike my previous theory, it doesn't explain why the hospital would give such terrible advice to Yuki regarding her breech baby. The only real reason I'm going with this is because granny seems to be involved in every major incident, and she seems to be what's tying all the stories together.