yeah I remember, but if you wanted a mind*** you wouldn't even write anything in red :P I'm just divagating here.
Spoiler:
Ok, you asked for a theory, there you go.
Let's suppose Natsuhi is the culprit. What motives she has are still a little unclear. We know that she believes in the "god of the shrine", and that she wants to carry on the epitaph, because she believes money won't do any good to the four siblings, and that they should not fight. the one that passed her belief on this to her, could have been Kinzo, for example. Since she's nearing the end of her life, she gets the "help" of her (and Kinzo's) most trusted servants, Kumasawa, Genji and probably Gohda. When Jessica goes to her room, she tells her of her plan to "save" the now corrputed family. Jessica decides that she wants to help her mother, because, to some extent, she also believes in the god. this seems a little far-fecthed, but could be possible. Also, if we consider Rosa is an accomplice, then the timing can be explained easily. First, she went from the guest hotel to the main building and drugged the tea. Then, she could have lied about seeing Krauss and Kinzo. Well, she did see them, but only after Krauss was killed. where they went, I can't be sure, because after that, Rudolf, Eva and Nanjo stay in the main building, so they couldn't hide there.
that's all for now, just a small drabble, nothing really too much, but it's also full of incoherences, I realize. well, it's time for us to start thinking up plausible scenarios. If I may, I would advice you to try finding what works where before we start thinking about any real motives.
Meaning, when could x do y, for example. Like "Who has access to the keys in the servant's room while Eva and Rudolf are discussing the "wolf and Sheep" theory?"