Let’s examine why Natsuhi’s death in the second gameboard is impossible.
Natsuhi died in a bathroom that has only two entrances, the door and the window. We know that Natsuhi indeed died there, as it has been confirmed in red that Natsuhi was dead when Kanon examined her corpse. It is impossible for people to convincingly disguise themselves as other people without proper foreshadowing, as per Knox’s 10th. In the absence of proper foreshadowing, we can only assume it was truly Natsuhi who entered that room, meaning she was alive before she entered the bathroom. As there can be no hidden passages in the mansion, and Kanon found no other entrances or exits, we can be certain that the door and the window are the only points of entry.
Kanon searched the bathroom before Natsuhi entered. As no details are given on exactly how Kanon searched the room, we must assume that whatever the culprit might have done, it must work out the same regardless of how exactly Kanon searched the room, as long as it is some reasonable method one could imagine Kanon using to search the room. As such, in the absence of anything in the narrative suggesting it is possible Kanon missed something or someone, we can only assume that there was nobody in the bathroom other than Kanon when he searched it. Someone entering the bathroom in the small gap of time between Kanon finishing his search and Natsuhi entering the bathroom has been denied as a contrivance not supported by the narrative. This means that directly after Natsuhi entered the bathroom, she was the only person there.
After Natsuhi entered, Kanon closely guarded the locked door, and nobody could’ve gotten past him unnoticed. Therefore, nobody could’ve entered the bathroom through the door in order to kill Natsuhi at any point in time. To put it in other words, no third party could enter the bathroom without entering through the window.
The window was locked when Kanon searched the bathroom, and it has been confirmed in red that it was still locked when said search ended. It has also been confirmed in red that the lock can only be unlocked by someone inside the bathroom doing it directly by hand. As only Natsuhi entered through the door after Kanon’s search was completed and nobody else was in the bathroom, we can conclude that the window was still locked when Natsuhi entered the bathroom. It has been confirmed in red that Natsuhi did not touch the window. The window has not been described anywhere, therefore we can assume that whatever trick makes the murder of Natsuhi work, it works regardless of what kind of window the bathroom window is, as long as it is a fairly ordinary window. For a fairly ordinary window, you would need to touch the window to unlock it. Therefore Natsuhi did not unlock the window. As Natsuhi was the only person in the bathroom and the window can only be unlocked from inside the bathroom, the window was never unlocked.
Due to the aforementioned red truth about not touching the window, we can also be certain that Natsuhi did not leave through the window. As Kanon didn’t notice her leaving through the door either, we can conclude that Natsuhi never left the bathroom, just to be sure that the scene of the murder was indeed the bathroom and not some other location.
As the window can be expected to be a standard window, in the absence of anything in the narrative suggesting it is broken in any way and a guarantee in red that the window was not tampered with in any way, we can thus conclude that it was impossible to enter the bathroom through the window.
And thus we have arrived at the conclusion that it is impossible for a third party to have been inside the bathroom when Natsuhi died. As accidental death has been denied by a red declaring that Natsuhi’s death was murder, we must inspect the possibilities of suicide and remote murder.
The rules of the game state that the culprit is defined as the one who kills. There can be only one culprit and no accomplices. Thus, if Natsuhi killed herself, she is the culprit, period. After Natsuhi’s death, Kyrie and Hideyoshi died under circumstances that cannot be the result of coincidence or traps. They were holed up in the parlor, and one lost her head and the other was found with a sword stabbed through him. No accidental death could plausibly cause this. It is likewise difficult to imagine a trap that could do so. Furthermore, after Natsuhi’s death Kanon met with a mysterious individual wearing a raincoat who gave him the skull key. This person would somehow have to be a person unrelated to the murders if Natsuhi was the culprit. In addition to all that, Kanon’s death would’ve had to be the result of a trap as well. All these together seem like such an implausible coctail of randomness that I believe it is safe to say that Natsuhi did not commit suicide.
Then the only option left is remote murder, “remote” being defined by any means of killing that don’t require the culprit to be in the bathroom. Red truth confirms the cause of death was a stab in the forehead. Furthermore, traps have been ruled out by red truth, with a trap being any kind of mechanism, timed or not, which would allow the culprit to kill Natsuhi without being in the same room as her. So, no mechanisms can be used to commit the stabbing. Red truth has confirmed that there are no gaps anywhere in the bathroom walls sizeable enough to stab a knife through, with the exception of the window and the door when they are open. However, neither of those could’ve been opened. In the absence of foreshadowing that the culprit could operate the bathroom facilities from the outside, and considering the position of Natsuhi’s corpse, we must assume that Natsuhi was killed after flushing the toilet and opening the faucet on the sink, but before she was done washing her hands. Thus, she would’ve been at the sink when she was murdered. If no mechanism was used to stab Natsuhi, the culprit must’ve somehow stabbed her through the wall. However, it seems that would run afoul of the red about no hidden passages, the red about there being no gap through which a knife can be stabbed, and Knox’s 8th. So, it seems like remote murder doesn’t work out, either.
Therefore Natsuhi’s death is impossible if the reds are taken at face value, which is how I understand DWaM’s game is meant to be played. Rebuttals are welcome.
Might as well take a look at something else, such as Rosa’s room. I expect this to run into Knox’s 8th, but I don’t have anything better right now. The door to Rosa’s room wasn’t actually locked. Instead, the door had been tampered with or otherwise jammed so that it would resist being opened even if it was unlocked. One way to accomplish this while still allowing the culprit to leave the room would be to lace the side of the door with glue.
One strange detail about Rosa’s room was that the key was on the floor. Considering how deliberate and theatrical setups the culprit seems to prefer, after going through the trouble of arranging all the corpses and such, just unceremoniously dropping the key on the floor and calling it a day seems odd. Which appears to suggest that the culprit couldn’t control where the key goes, meaning it was somehow smuggled in after the culprit had left the room?