I still think the 3 suspects are red herring. If Uketsu is keeping their style from Strange House, then it's possible that the murder motive is simply something completely irrelevant to Miura himself. Kumai posited two reasons why Miura was mangled badly, either to hide his identity or because of a grudge. Kumai said it should be the latter because Miura's ID was left on the scene, but think about it. The culprit, according to Kumai, has enough thinking to steal the sandwich as well as the sleeping bag to create a fake alibi, but in that case, why don't they take Miura's ID too? Why did they half ass it? Even if the culprit assumes the police will fall for the "staying overnight" misdirection, the police will still pinpoint the grudge perspective due to the mangling, and thus narrowing the suspects to the three people.
And in this case, Toyokawa's alibi is especially too flimsy in that it's too good to be true that he is the culprit. He has no alibi at all from when he split with Miura all the way until 7 am the next day, meaning he basically nearly doesn't even make it for the B type alibi. Also, according to Kumai, he barely has any motive and him being invited to the hiking was an impulsive decision from Miura's part on Saturday's night. This means in the span of less than a day, Toyokawa needs to have planned about going through the animal trail to reach the 8th waystation as well as about the fake alibi misdirection, but in that case, why did he fail in even making a better alibi, only showing up at 7 am the next day? And if the murder is truly spontaneous without planning, if he has enough time to think about stealing the sleeping bag and sandwich, why didn't he even think about stealing Miura's ID card, wallet, or even the whole backpack? The easiest way to obfuscate a murder is to make it seem like the motive is from robbery.
At this point, it somehow becomes a bit too 'obvious' that the suspects are among the three, especially with how 'easy' it seems that the suspect is narrowed on Toyokawa. In this case, I propose three possibilities.
First, is that the culprit intentionally mangled Miura and took only the sleeping bag exactly to lead the police into assuming the murder is motivated by grudge and that the culprit is trying to create fake alibi. In this scenario, the culprit is leading the police to narrow their suspicions onto the 3 people while the culprit is actually someone else. This possibility is based on the assumption that Miura was killed because he saw something he didn't suppose to see, and that the culprit is probably a crazy psycho or someone with anger issue. After murdering Miura, they took the sleeping bag and sandwich to obfuscate police's investigation.
Another thing that to me supported this possibility was that Miura's corpse wasn't buried. Because again, if the culprit has enough thinking to do the whole B-type fake alibi thing, they would have thought about just burying the corpse altogether to make it even harder to narrow the suspect. Ergo, the fact they didn't bury the corpse is to make sure someone will find it, and then to eventually come to the same conclusion that Kumai made in this chapter, which is the actual misdirection.
The second possibility is that the whole thing is accidental. Miura saw something he shouldn't, which then resulted in him getting killed. The murderer, realising they had just murdered someone, then took Miura's sleeping bag and sandwich in a panic and actually slept on the mountain in fear of being found out before slipping away in the morning. This fits Uketsu's style from Strange House. In Strange House, the handless kid was thought to be forced to become a murderer by the protags, only to be revealed in the end that that wasn't the case at all. Similar thing might happen here. The murderer might have done all this stuff while panicking and all in order to hide themselves in the mountain, only to realise that they did manage to get away with it out of sheer dumb luck because the police was being too smart when investigating the whole thing.
For this second possibility, the unburied corpse is not buried simply because the culprit ran away in panic after realising they murdered someone. Plain and simple, the kind of schadenfreude that I can see Uketsu making.
The last possibility, of course, is that the corpse isn't Miura at all. In this chapter, Kumai said that the only reason the police found out the corpse was Miura's was because of the ID card, the backpack, and the left behind car. But when you reverse that line of thought, if not for those things being left behind at the scene, the corpse wouldn't be identified as Miura at all. As such, there exists a possibility that the corpse isn't Miura's but someone else's.
In this possibility, Miura probably saw something he shouldn't and got knocked out. Something that he was probably another murder attempt or someone on the way to being murderer. After knocking out Miura, the murderer had the idea to switch the identity. The murderer's original victim was forced to eat Miura's leftover bento, and then they were killed after 2.5 hours had passed, thus creating the illusion that Miura was killed on 5 pm. The corpse was then mangled and left out in the open, with Miura's belonging planted on the scene, thus perfecting the illusion that this corpse was Miura's and that he was killed due to grudge. Miura himself meanwhile, probably still alive until the night of the 20th, but the murderer might have discovered him trying to make the dying message (the grid drawing) and thus killed Miura as well. Just to make sure, the culprit also planted the grid drawing on the corpse, which also helped to trick the police into thinking that the corpse was Miura.
What about Miura's corpse then? Probably actually buried somewhere else on the mountain, never to be found even until today.