"BEATRIIIIICEE~" So my Umineko package arrived yesterday, and I've been speed reading through Legend of the Golden Witch again to unlock the music box and time traveling for that episode. Anyway, a possibility occurred to me:[spoiler]Though I still subscribe to my Logical!Beatrice theory, as long as we're supposed to be discussing totally human alternatives, I have reason to suspect Eva plus one other killer. Eva's already demonstrated her niche for martial arts, telling Rudolf she could knock him unconscious in a single blow. That said, tempers flare, she'd take down one siblings "Accidentally," the rest hesitating to react—thinking she'd just succumbed to the heat of the moment—when down goes another, and possibly another. Hideyoshi may or may not have played part, in which case it becomes Eva plus two killers. After eliminating the four (...or is it
three?) in the dinning room, tracking Gohda and Shannon individually would be simple, though one of them was probably grabbed spur of the moment when they couldn't touch Natushi.
The Anonymous Killer would then later backstab Eva (and Hideyoshi?) when they were separated from the remaining family, and presumably safe from her second (third?) partner.
Eva is overall the least distraught of all the family members when they discover the shed (and Hideyoshi already has a penchant for acting a role), and still has the clarity/forethought to do stuff like stick a receipt in Kinzo's door to frame Natushi (who, again, she couldn't touch).
Reasons for leaving Natushi alive would be both/or (1) maintaining the illusion of the summoning ritual, leaving her alone because of the scorpion charm hung on her door, and/or (2) requiring a scapegoat to pin the crimes on, as Eva failed to do with the receipt.
The Anonymous Killer then takes out everyone after Eva and Hideyoshi are disposed of, as well as probably/possibly murdering Kinzo.[/spoiler]