Hi! Higurashi anon here! It has been a while but here I am with my theories for the Question Arcs!
For Onikakushi, I really think Keiichi went... a bit... paranoid (well, very paranoid) and that things weren't really as much as he saw. By the end of the Episode we even see hints of him questioning whether some things happened or not. This doesn't mean everything was just his imagination. I definitely think something was happening, but he wasn't very objective. The syringe was definitely real, as it's even mentioned in Watanagashi too. It definitely had some kind of drug... and the fact Keiichi repeats what Tomitake did to kill himself at the end definitely prompts me to think he was somehow drugged with the same drug. But I do wonder what's the thing with the syringe... and the fact that of everything Keiichi wrote in his letter, only his theory of the first victim actually living was taken along the syringe. Onikakushi is really mysterious in the terms of the Whydunnit, but I do think everything can be explained in human ways. Keiichi wasn't on his right senses, he kinda hallucinated many things, and Ooishi wasn't helping either... And if he was drugged then as well...

Watanagashi is pretty direct, and only introduces its big mysteries at the very end in my opinion. The last scene in the hospital... Ooishi asks Keiichi if he really was stabbed by Mion, revealing that it couldn't be because Mion was actually dead by much earlier.
Of course, and this is when my big theorizing starts, if you want to explain this in a human way, there's only one possible option. It was Shion. Shion disguised herself as Mion, acted as her and stabbed Keiichi.
Now, the interesting part for me was the Whydunnit. Because this Shion states something very interesting after stabbing Keiichi. That she has finally killed everyone she wanted to kill.
Of course, there's the option of Shion having suffered such a big trauma that she, being Mion's twin, tried to finish what she started as her... but I don't really think so. And it was from that statement that I went to another theory with a single question: "At what moment had Shion suplanted Mion's identity?"
Was it really only at the end? Or it was... much earlier?
There's a lot of focus on twins in this episode after all, and how hard could it be do distinguish them... close to the end there's also some Hints about Mion and Shion's relationship, how weird was that the Sonozaki allowed twins instead of getting rid of one, etc.
So my theory is ultimately that Shion kidnapped Mion and then supplanted her, probably after the Watanagashi. Aaaaaand here comes another big hint for this! "Mion"'s reaction about Satoko and Satoshi when they went to look for Satoko and Rika. The way she answered, triggered, blaming Satoko for Satoshi's disappearance, saying she's cursed... especially after Tatarigoroshi, it becomes super obvious in my opinion. That was Shion dropping the act for a moment.
I think Shion could have been in love with Satoshi, and his disappearance was a big trauma for her. Considering how she talked in Tatarigoroshi and the especific deaths in Watanagashi, I think Shion could have thought the Three Families killed Satoshi? And designed a plan of revenge to kill their leaders, leaving Mion alive till the end so she could make her suffer the most. So she killed Oryu for the Sonozakis, Hinamizawa' mayor (leader of another family, I don't remember the name) and Rika for the Furude. She also tortured and killed Satoko for the same...
Idk why would she mindtorture and try to kill Keiichi though. Maybe just because he's Mion's friend? Because Mion seems to like him? Because he reminds her about Satoshi (as Higurashi really likes to mention how similar they are) and she can't take it? Mmm
Ultimately, in the torture underground, Shion would force the real Mion to act like she was Shion in front of Keiichi, then she would kill her and escape quickly afterwards, changing to Shion again.
But maybe because of guilt or lack of hope or something, at the end she commited suicide after stabbing Keiichi... if i remember well someone witnessed having heard her like talking with herself or something? So she maybe wasn't in her right mind after all of that...

Okay! Watanagashi's second great mystery! TAKANO. At the end Keiichi receives the big reveal that they couldn't have been with Takano during the Watanagashi because surprise, Takano had been dead for one or two days before that. BUT both Keiichi and Shion saw her. They were definitely with her.
So, who was that Takano? Once again, if we go with human theories... the answer is that they definitely were with her. So the corpse found had her details faked and wasn't actually Takano, or the Takano they met there, and during all the game, was a different person using that name.
But I'm going to leave that for later.

Tatarigoroshi! What happened here? I think I can explain it more or less. The one thing I can't really explain would be Keiichi somehow managing to survive the Great Hinamizawa Disaster, as the last scene reveals that he couldn't have survived if he really had been there the whole time. Unless... well, he was lying for some reason, but idk.

Okay, so... Keiichi really murdered Satoko's uncle. I don't have any doubt about that. In the hints, it's revealed Ooishi and the police were thinking he could be the victim for this year, so they observed his movements. They confirm Satoko's uncle left the house and never got back. The bat is gone as well, and so is his bike.
So... what happened? Why would Satoko say that he never left and was there all along?
In any case, Satoko couldn't have been telling the truth....
But I really don't want to think like that Hint where it's said you shouldn't believe her just because of what happened with her stepfather. I think Satoko's trauma was so big that even after her uncle left... for her, he was still there...
What happened with the corpse? Someone had to have moved it. I thought about Takano who definitely seemeed to know what Keiichi had done. I also thought about... well, Satoko. It's interesting to know that Satoko was going back home during the same time Keiichi was killing her uncle. So the possibility of her witnessing it and trying to help Keiichi (as we know Satoko always helped Satoshi) could be a chance, and we know she can move perfectly in the forest, but mmm... she moving a corpse, dead weight... without help... idk. Even then, this would contradict the end of the episode with Satoko trying to kill what she think was Keiichi possessed by something because of her. (though maybe she could be triggered because of Rika's death, but I don't know)
Takano moving the corpse to help Keiichi could just be a way to protect herself as... well, I'll talk later about that.

Who was the other Keiichi? I thought about it when it happened, because of something similar happening in Watanagashi, and Himatsubushi basically confirmed it for me. ...I think.
Everyone was covering for Keiichi, they were lying. Himatsubushi puts so much empashis in this. That in Hinamizawa everyone could organize so there were no culprits, especially under orders of the Sonozaki (with Mion... or Shion supplanting her too? because of the phone talk where she says Keiichi is acting like Satoshi). So I think it's that since Mion basically knew what Keiichi was going to do.

Ooishi's disappearance just after asking to identify a car... I think there's only one car important in this episode. So the owner appearing after finding the car could be related to this...

Irie's death... Idk why would he do that, or if he was forced. (Maybe?) Takano's death can be explained the same way as in Watanagashi. I definitely think she's actually alive.

I really can't explain the Great Hinamizawa Disaster in a human way apart from coincidence... unless somehow Keiichi sinking the bike in the swamp could have caused it lol
I do wonder if the disaster also happens in Onikakushi and Watanagashi... An interesting detail about the Disaster is that in the list of victims, Satoko appears as missing. She wasn't found, just like her uncle. What happened to Satoko...? This is just a theory, but along Keiichi's mysterious survival, I wonder if I'm supposed to believe him. What if Keiichi's POV in this episode wasn't really objective because he's trying to cover for Satoko to protect her? What if something different happened during the climax, and somehow Keiichi managed to save Satoko from the disaster? Idk.

By the way I find interesting the parallel with Umineko. The Great Hinamizawa Disaster, the Rokkenjima Incident with the explosion... things that are introduced close to the end of the Question Arcs, at the end of the third episode (though in Umineko the explosion isn't really mentioned until Erika's dead profile at the end of EP6, but you could kinda guess something happened from Rokkenjima's description when Ange visited it in EP4)... the possibility of the Disaster being something caused by humans and not just a coincidence is also discussed in Himatsubushi. If it was like Umineko, maybe there's some kind of device able to pull it up too - though it seems far fetched.
Hahaha. Maybe the person Lambda turned into a "god" was Satoko, by giving her the certainty of the "disaster device" if something like that existed, just like Lambda gave Sayo the certainty needed to create the catbox with the bomb. Ok now I'm just rambling nonsense, sorry. Please don't take anything of this seriously.

And finally we reach Rika's death in Tatarigoroshi, and with it, the Final Mystery of Himatsubushi.
Who kills Rika? How and why? Who is the Big Culprit?
My answer, although a bit obvious after Tatarigoroshi: it's Takano. Or whoever this person is if they're just using that name without being her.
Like, really, once you try to justify her being alive in Watanagashi at a time she should have been dead and the circumstances of said death... it's pretty obvious she's involved in something big. She's too suspicious. She also expressed interest for Hinamizawa, its people, culture and legends, and there's a Hint just before Keiichi and Satoko finding Rika's corpse about what would happen if Rika, the incarnation of Oyashiro-sama, were to die, with Takano (I think it's her at least) finding the scenario very interesting.
And I mean. Tatarigoroshi all but explicitly say Takano murdered Tomitake. And since he always die in the same way in every episode... that would mean Takano is the one murdering him in Onikakushi and Watanagashi as well. Though I wonder why she disappears only in Onikakushi, faking her death in the rest of episodes. In any case, had Takano survived in Tatarigoroshi, she could have killed Rika. There's also the fact Takano's car is the only important one that appears in this Episode, and Ooishi and his partner disappear just after asking HQ to tell them about a car they had found. (Though in Himatsubushi, Ooishi is alive and apparently only his partner disappeared). To me it felt like a hint of Takano being involved with their disappearance as well.
Since we're on the topic, Takano was said to work for Irie, I think? At least in the same clinic he has. I wonder if she was also involved with his death.
Irie and Takano... the clinic... I don't think Irie is the culprit, though maybe he could be an accomplice...? mmm.... idk
In relation to Onikakushi, I do wonder if Irie was the "director" Rena and Mion called to "help" Keiichi.

Well. Right now I think that would be everything. I'm sorry since I talked a lot! There's a lot of things and possibilities... I really think Takano is the big culprit killing Rika for some reason. Thinking about it, considering she made Keiichi think the Sonozakis were behind the previous murders, I do wonder if she could have also made Shion think like that in EP2, making her act so she would kill Rika at the end as well without Takano needing to do anything. Buuuuut Rika did survive in Onikakushi, right?
Why is "Takano" murdering Tomitake? Why is she so creepy and faking her death? Why would she want to kill Rika apart from mere curiosity based on Hinamizawa's legends? What's the whole thing with the Disaster...? Mmmm....

Well, next episode is Meakashi! First Answer Arc, and I know Shion will be the protagonist. So at the very least maybe I'll find whether my theory about her is correct.

Thank you for reading this! 😭