If there are no accomplices, Battler acted as though he had no idea where the key had gone from his pocket, and it is impossible for him to drop it or for someone to pickpocket it from him without Battler noticing, what other conclusion is there to draw other than that Battler is the culprit? Explaining the key in the pot and the HA HA HA is tricky without a culprit who isn’t a part of the group, though.
Xak made a blue explaining the locked room in Rudolf and Kyrie’s room, and DWaM didn’t refute it in red. But even though he said it is possible enough, I really can’t see it. An adult can’t fit into an adult’s dress without their head sticking out or it being immediately obvious something is up. If the chairs they were sitting on were sofas or somesuch, the culprit could’ve cut a hole to hide their extra mass in, but the word “chair” to me implies something that doesn’t have that extra space to carve into a hiding spot. Even if you argued that Kanon is 16 or something and small enough to pull it off, the theory encounters a problem. If Kanon’s body is indeed so notably smaller than that of the adults’, then it should’ve been immediately obvious to the people finding the charred corpse of “Kanon” that he suspiciously grew a lot bulkier during the night. Among the dead, there should be nobody who is of similar size to Kanon (if we assume he’s small enough to be able to pretend to be the headless corpse of Rudolf or Kyrie), so a substitute corpse couldn’t have been used, right?
If the above is true, then an alternate solution must exist. I’m inclined to believe it must hinge on the chain, since should there be a trick involving the windows that leaves no traces, the culprit could’ve used that with the previous locked room as well without having to resort to burning ropes or such. I can’t think of any difference between the two locked rooms that would make some window trick possible in one but not the other. Thus, the trick must be based on the one noticeable difference, the chain. Perhaps something like this? The culprit had broken down the hinges of the door in advance. He set the chain from the inside, and then used glue or some other implement that allowed him to “fix” the door from the outside. Even if the glue didn’t hold the door in place all that strongly, nobody would’ve noticed anything since Battler, the culprit, was the only person to touch the door before it was broken down. As long as the door provided some resistance, it is plausible enough that Kinzo wouldn’t notice enough of a difference to suspect someone had tampered with the door in advance. If he was able to kick down other doors in the mansion, one can assume that even the doors that are held in place by proper hinges aren’t that durable.
The key in the pot is a problem. As the culprit, Battler would’ve been able to make sure that he’s the one who finds the key, and he could’ve prepared a trick in advance that lets him put the key inside the pot, whatever that trick might be. But I wonder why he would even do such a thing. Having the key mysteriously vanish from his pocket while assuring that there’s no way anyone could’ve stolen it or that he could’ve dropped it just seems like a way to make himself seem more suspicious. Not to mention that he tried to go to the pots. If Kinzo hadn’t stopped him, the key trick would’ve lost all value as something Battler couldn’t possibly have pulled off. It could be he counted on Kinzo stopping him, but that seems like a pointless risk. In any case, the trick. An unsatisfying solution would be that Battler threw the key in after the lid was removed from the pot, and somehow nobody noticed, but I don’t have high hopes for that theory. A more convoluted theory would be that Battler dropped the key down from the window in Eva and Hideyoshi’s room. He had prepared in advance a slide that stuck out from the kitchen window that would guide the key inside the pot, and a string that he could pull to move the lid to cover the pot fully. The string wasn’t tied to the lid directly, it merely nudged the lid when pulled, and thus didn’t leave traces in the kitchen. However, this would require the narrative to omit two relevant details - that Battler went to the window in the first place, and that the kitchen window was open. I suppose Battler could’ve investigated the room a little when Shannon was chasing down George. If only Battler had gone vomit through the window.
A less plausible alternative to the above is that Battler had prepared some strange trapdoor in the floor outside Eva and Hideyoshi’s room, and used his vomiting break to covertly drop the key to the pot and put the lid in place, perhaps with a string trick.
When it comes to the HA HA HA, I don’t really have any explanation, though. Something like the text being written there in advance but covered with some substance that vanishes over time should be eliminated by the red about the text not existing when Shannon and George searched the third floor. I don’t think there were any opportunities for anyone in the group to go paint the study door, so this, at least, seems to require a culprit who is not a part of the group. Yet, the reds seem to leave no room for Battler to be innocent because of the key. Might as well go ridiculous and say Battler had prepared a huge stamp with AH AH AH on it that he’d hidden behind a trapdoor on the roof. A timed mechanism was set to open the trapdoor and hit the door with the stamp, and then retract the stamp and close the trapdoor. This caused the HA HA HA to appear in the door when nobody was looking. Only Battler could think of a method as stupid as this, thus Battler must be the culprit.