Ah, Bern's game. It's very fun. I couldn't do it.
This isn't talking about the number of culprits, but the method of murder. Anyone considered a culprit must commit murder directly, by their own hands. No traps, no proxies, no accidents. If someone is dead, then the method was perfectly straightforward.
Correct. By the end of the game, if someone committed murder, then they are alive.
Don't worry too much about Shannon and Kanon. Their dual identity is irrelevant to Bern's game.
This is because it was written by Bern. She has no love, and she doesn't care. Everyone talks unnaturally because Bern doesn't care enough to write them as people, she writes them as props, things to move the story forward and nothing else.