I’m thinking that the vents are a big clue to solving this mystery, as well as Krauss’s useage of the storage room.
We know from the room layout, that the storage room is connected to the cousins room. The wound on George’s hand supports that theory as he stuck his arm into the vent below and got a gash for his troubles.
Krauss has the weakest alibi as he was in the storage room looking for something. What that something is isn’t brought up by him in White or Purple text as nobody questions him on what he was looking for exactly.
Then there’s the bruising on Rudolf’s body as well as his clothes not being soaked. The bruises could be explained by someone punching him hard enough to create the bruises. The clothes not being wet may be from using the hidden passage hinted in the letter, in which the weather outside wouldn’t affect the clothes.
I’m aware that this theory may have a few decent sized holes, but that’s the best I have for now.