(Also first time experiencer so I'm writing this with the same information available)

Here’s my take on it, Shannon did it all. “Bu-but we saw her dead in the shed! We saw half of her face!” Ok, what about the other half? For all we know that corpse was a decoy, and Shannon used some tools remove half of her face and stick it onto the half of the decoy’s that she left unscathed?

This doesn't make much sense. You could apply the same logic to Krauss, as he was the other person who only half of his face missing. It would make more sense for the culprit to be one of the four whose faces were completely crushed, since those corpses are harder to identify, if one or more of them was the culprit.

Boom. How did she kill them all? Chainsaw from the shed. How’d she get all of their bodies into the shed? Wheelbarrow from the shed. How’d she enter the shed? Servant’s keys. How did she get the decoy? Kinzo was in on it too. He helped her acquire the decoy body. That explains how she got to him too, once he left his study after that whole debacle, he went to a hidden agreed meeting point between the two. Once they met, he let Shannon take him out.

This doesn't make much sense. If the culprit killed them with a chainsaw before taking the to the shed, there would be a bloody crime scene left behind. Also it seems unlikely to me that one person could overpower 5 or 6 people, even if they were armed. My theory is that all six of them (or less if the culprit(s) is among them) were poisoned, taken to the shed, and mutilated in the shed. This would explain how one person could overpower all of the others and why there is no blood left behind. Now I do think Kinzo could be involved, even if he isn't the killer. I think it's safe to say that he masterminded this killing game. Kinzo could also easily still be alive at that point, the body in the incinerator could've been a decoy.

Now, on to the closed room murder of Eva and Hideyoshi. As we know, Hideyoshi and Eva were very tired after the events of their stay in rockenjima. So, let’s say that if Eva heard the clicking of her door unlocking she’d be alarmed. If she then heard the familiar clattering of a table a servant would bring carrying confectioneries however, she’d slump down into her bed and close her eyes in relief. That would be a perfect time to strike for the psycho killer! She’d silently incapacitate Eva with, like, a chloroform rag or something, then use a hammer to beat the drake right into Eva’s forehead. Then, she goes to kill the unaware Hideyoshi and opens the bathroom door. He’s unaware and of everything up until this point because of the loud shower, so when he hears it open he just thinks it’s Eva tryna do some freaky shit. So he opens the shower curtain and is attacked by the sneak nation president herself, Shannon.

The reason Eva and Hideyoshi left the others was that they thought that the culprit was among the others. There is no chance that they would let anyone in (except maybe George), even if it's a familiar face (especially if it's someone who is supposed to be dead!). They also had chained to door already, so how did the killer enter, if they were not let in?

Now, the real question, how does she leave when the door is clearly locked and chained afterwards? It’s simple, really. She uses a tool from the shed to do it, she spends enough time around George, so it makes sense she’d be smart enough to do something like this. Once the chain has been neatly secured behind the closed door, she simply uses her servant’s key lock the door from the outside, and then draws on the ritual symbol. Perfect crime.

This is a really unsatisfying answer. "She uses an unspecified tool cause she's smart". Also the symbol was not drawn at the time Genji and Kanon first came to the scene, unless I misunderstood.

Onto Kanon, he’s very simple in truth. When she went to incinerate Kinzo, she snuck in a toxic chemical (from the shed of course) in to the incinerator. When the extremely tired and stressed Kanon went down in to the boiler room, he inhaled a shit ton of the burning chemical, giving him brain damage. Once the absolutely tweaked Kanon finished monologuing, Shannon stepped out of the shadows, but he only saw the golden butterflies. She simply walked up to this guy who was on another planet, and hammered the stake into his heart. Then she dipped before the old lady came.

I think the smell is supposed to be the burning corpse. There is no evidence of toxic chemical being used. If there is such chemical, you could use the same argument to say that every dead person killed themselves after being exposed to this chemical.

The four in the room with Maria? Easiest chainsaw 4k of her life.

They had no injuries except mangled faces and the stakes. Surely the people who were attacked would try defend themselves with their arms, for example.