Alright. Let’s break down what, from my understanding, is every mystery in play here. If you’d be so kind as to point out any unaddressed mysteries that would be necessary for a solution, I’d appreciate it.
[color=blue]The culprit is Rudolf. The body in Kyrie’s room is a prepared fake dressed in his clothing. Alternatively, it is Gohda or Genji’s body dressed in his clothes. [/color] This theory works for any of the adults or George playing dead, so I’d just repeat it all with the name substitution 
Natsuhi’s Room Key
[color=blue] The culprit can move freely, and thus planted the key once Kanon had left the area [/color]
Rosa’s Locked Room
[color=blue] The culprit locked Rosa’s room from the inside, and hid in the desk as Blackrune theorized. In the case where he used Gohda or Genji’s body as a decoy, Rudolf dressed the body up as their original owner and put it in Rosa’s locked room as well. This could’ve occurred while the group discovered Natsuhi and Krauss, and could have been the reason he directed them to that room. [/color]
Kinzo’s Stupid Box of Poison Adventures
A few possibilities here. We have it in red that Kinzo would not have let anybody into the study of his own volition after showing Kanon the poison box the night before. This means the culprit had to have used the key to the study to both steal the poison and kill Kinzo.
[color=blue] Kanon is the key keeper. He was supposed to return the key to Kinzo when he delivered the coffee, but he forgot to. Kinzo realizes that Kanon should still have the key, so he is highly suspicious of him during their encounter outside his study. Kanon believes he dropped the key off, and thus only Kinzo could have let someone into the study. In fact, Rudolf pick pocketed the key from Kanon while he slept. Thus, both Kanon and Kinzo’s actions make sense. Rudolf could freely use the key to steal the poison and kill Kinzo whenever he chose to. [/color]
It is highly likely that Kinzo’s study is NOT soundproof, since they can communicate with him through the closed door. This means that Kinzo may have actually been shot the night before, since nobody heard a gunshot after the interaction with “Kinzo” through the door. Battler heard a gunshot from the ground floor that presumably took place on the third floor, since it seems like that would be Kanon getting rekt. This points to the idea that a gunshot would have been heard from anywhere in the house, except maybe if it took place from within the library.
This would mean that if [color=blue] Kinzo was shot the night before, and the culprit created a “soundboard” like recording of Kinzo’s voice, which he used to trick the group into thinking he was still alive. [/color] This doesn’t contradict the red of the voice actually belonging to Kinzo and actually coming from the other side. Both of these blues can coexist together anyway.
The Power Outage and Nanjo’s Death
Again, the culprit can move freely. No problem here.
Natsuhi’s Worst Bathroom Visit Yet
Let’s stick with one of the theories that passed earlier. The remote mechanism tied to, lets say, the tap.
I’ll note here that Rudolf is one of the few people who could’ve still been alive to kill Natsuhi. Krauss, Shannon, Kumasawa, Genji, Gohda and Rosa seem pretty convincingly dead. Jessica was dead before all the killings finished this time around (proven in red), so it’s unlikely she can be the culprit. Kyrie and Hideyoshi have an alibi. Battler is… probably not the culprit. There’s too much working against that idea both in this game and in the last. Kinzo is dead, as confirmed in red. That leaves one of Nanjo, Rudolf, George or Eva faking dead. Do let me know if I’m overlooking someone here.
HA HA HA
Rudolf would have been free to write this graffiti as well, then meet with Kanon (after doing this:)
Kyrie and Hideyoshi Dead in the Parlor (with the sword)
[color=blue] Rudolf uses the sword left by Shannon’s body to kill Hideyoshi. The only way Kyrie would let anyone in to the parlor is pretty much if they’re Rudolf come back from the grave. Which he was. She lets her guard down and is knocked out by Rudolf, who then impales Hideyoshi. He takes Kyrie’s shotgun and beats her to death with the butt of it. [/color]
Kanon Gets Shot
Kanon gets shot.
