Natsuhi's death in the hall

She was shot by Sayo, who is still alive unbeknownst to us. Both the Shannon and Kanon deaths were faked using tricks that don't involve the use of unidentified corpses.

Leads:

• The reds seemingly contradict each other, especially after we learn the culprit was present in the same room (thanks Lambdadelta!), so something has to give, and There were no unidentified corpses, and all of the survivors have alibis! is the only part with some wiggle room.
• We are forced to restrict the definition of survivors to include only the people we are aware of at that moment, and not any other potentially alive person on the island.
• The unidentifies corpses part only rules out some tricks, but it's still possible to lie about someone's death without showing the corpse.
• We know from later episodes that there are multiple winchesters.

Natsuhi was shot at some point previously, maybe during the murder of the first twilight, and from that point forward somebody else impersonates her. At the end her actual corpse is taken out

Leads:

• It explains why she would run away from the parlor.
• It solves the red with no extra people at the end.

Obstacles:

• We ran out of possible candidates.
• If our culprit X is Natsuhi who is always with battler at the end, who's killing people?
• Convincingly passing as somebody else is quite hard when you think about it, you would need a lot of practice.