Posting our pre-trial theories here sounds like a fun idea, so I’ll be doing that when I get to the fourth class trial. I might as well also write here what I remember about my theories regarding the murders in the first three chapters, so here I go. I won’t be mentioning whether I was correct or not, though I suppose nobody who hasn’t played through the trials will read these.
Shuichi is the culprit. Assuming the cameras are reliable, the murder had to be done remotely, as the cameras captured only one opening/closing cycle of the hidden door, and the door was closing as Kaede and Shuichi rushed to the scene. This means that Rantaro died just mere seconds before they opened the door. Gonta could not be the killer because his line of sight to Rantaro was blocked by the hidden door being open. This means that either a trap was set, or Rantaro committed suicide. However, the hidden door camera had its flash on. This implies a trap, since Shuichi said he’d turn off the flashes from the cameras, meaning it was deliberately left on. As for why he would do this, the trap had to capture the victim’s attention to move him into the correct position.
It is not impossible for Rantaro to fiddle with the camera flash just to screw with people, but that seems highly unlikely. So, trap. Also, the shelf right in front of Rantaro suspiciously had books missing from the top. Our culprit could’ve set the ball there and then rigged some system to give it a nudge, perhaps using those quadrocopter things in the warehouse to allow remote triggering of the trap. Nobody but Shuichi (or Kaede, I suppose) could’ve set this trap, since the cameras would have captured them entering the room. However, this problem can be circumvented by Shuichi alone. When he momentarily left the classroom where he was waiting with Kaede, he could enter the library, allow their picture to be taken, simply replace the film, have a picture of their face taken, and run outside during the camera picture delay. Since only single pictures of each group of people arriving in the room were taken, there obviously is some delay between pictures.
Then, Shuichi would just obfuscate the order of the pictures and claim that the picture taken when he was leaving the room after setting the trap was taken when he was removing the cameras. Monophanie surprisingly grabbed the cameras and developed the photos instead of Shuichi, but that’s not a problem - she only said that she gave back all the pictures taken by the camera, not that she gave them back in the correct order.
There are some unlikely ways in which someone else, such as Miu, could have used the same plan, but those seem way less likely than Shuichi. However, I don’t quite know how to argue that Kaede is innocent. I’m pretty sure she’ll be accused in the trial, but the Shuichi culprit theory works pretty much equally well with Kaede, and I can’t find any solid argument for why Kaede couldn’t have done it. As for Shuichi’s motive, perhaps he was trying to hit the mastermind. The reason he would not use the First Blood perk would be because either he didn’t trust the mastermind to actually allow him to go free, or he wanted to use the trial to figure out the mastermind. In any case, he either wants everyone dead, or he’s ready to die for his plan, whatever it might be.
As for my considerably shorter thoughts for the second case…
I don’t really know who the culprit is, but Ryoma was killed in advance, probably during nighttime, by being drowned in the sink in his study. Then, the cable was used to transport the body from the lab window to the gym window, after which the body was placed into the piranha container. The square glass pane was used to separate Ryoma’s body from the fish. And that’s pretty much it. The timing of whatever happened when is a bit loose.
Suspects that seem the most likely include Angie, Himiko, Kirumi and Kaito. Though I’d be willing to strike Kaito off the list just because of how telegraphed it was by the narration going all “I trusted Kaito, without knowing how dangerous trust would be in this game…!” at one point. Angie contributed to the investigation with a helpful piece of information (cramped piranhas), but since that piece of information was pretty obvious, it could’ve just been an attempt to appear useful. Kirumi seems like the most likely person to take drastic action due to a motive video, as she’d probably fulfill a request from her old master to “return back to him” or somesuch, but the motive videos might as well just be a red herring. The narration about the dangers of trust could refer to Kirumi, but it could also refer to Himiko’s trust in Angie. It might also be that someone outside those four is the culprit.
And finally, chapter 3.
Korekiyo murdered both. The murder of Tenko using the seance was obviously planned and prepared in advance, as the floorpiece thing was cut to allow one of the floorboards to be tilted. Since Kokichi stepped through the floor in one of the other similar rooms, it appears the same preparations were done for all three rooms, negating the argument that Kiyo couldn’t have done it because he didn’t pick the room. The seance was Kiyo’s idea and involved such a convoluted process that it isn’t reasonable to think anyone else could’ve planned it in advance. It is technically possible that since Kiyo was open about wanting to try the seance out, someone researched it in advance and set up the trap assuming that Kiyo would want to test the seance, but that seems highly unlikely.
As for Angie’s murder, it was a coincidence. The dried blood in the floorboard and beneath the seance room can only belong to Angie, unless the culprit cut themselves for whatever reason. This means that Angie was actually attacked in the seance room. As for why Angie would go there, she wanted a candle to burn the Necronomicon with. She happened to come upon the culprit preparing his murder by chance, and the culprit silenced her by striking her on the forehead with the floorboard. This didn’t kill her, however - she only fell unconscious. The culprit bandaged the wound with duct tape, and carried her to her lab. There, he finished her off by stabbing her with the sword that was found in the effigy of Kaede.
The locked room was constructed with the effigies. The sword stabbed into the Kaede effigy was used to lock the sliding lock by rotating the effigy and quickly escaping the room, causing the sword to hit the lock and close it. It is a bit improbable, but since the murder happened at night, Kiyo would have had ample time to try it out until it works. The other effigies were hung from the ceiling to conceal the trick.
As for how Tenko was killed… dunno, really. The blood on the white sheet does not have a cage pattern on it, suggesting that it was prepared in advance instead of it being Tenko’s blood. That would make it Angie’s blood, which also suggests the culprit was the same for both of the murders. In any case, I can’t really think of a reasonable way the murder happened. My best theory is that the loose floorboard was used to have Tenko fall off-balance, turning to the side and thus getting her neck close to the cage wall, allowing Kiyo to lift the white sheet and strike Tenko with the sickle through the cage, but that doesn’t sound too reasonable to me. How would the culprit even trigger the floorboard yielding under Tenko at the desired moment, anyway? I also can’t really explain Kiyo’s motive for killing two people, when one would be enough and the coincidental murder of Angie would be much more difficult to solve than Tenko’s highly planned death.
Additional post-trial explanation for why I failed to explain a certain thing: For whatever reason, I had thought that the cut part of the floor was under Tenko so that the see-saw would work the other way around and make Tenko fall down, instead of the cut part being outside the cage and making Tenko rise up. Under the floorboards, it is clearly shown that the cut part is not on the same side where the bloodstain is, so I don’t have any excuse for this mistake. If I had understood the orientation of the see-saw, then I might have figured out the entire case in advance (well, except for the culprit’s motive).