Part 1: Bad Reasoning

It felt strange to assume the game room is a sealed room.

Huh?

It would be more reasonable if we assume its just Yoshikos delusion or there is more than one key.

Uh
Thats Sigmas reasoning

Yep. What concerns me is the leader of Kurogami being killed from behind, so I came to me, Shizuma wasnt attacked.

He used his death as the closing curtain to this incident.

What about other events?

Obabas death should be the poisoning from the meal. Shizuma probably poisons it while he murdered Yuriko. Obaba did said I happen to passed by and notice the meal is already prepared. Even if its related to Yuriko, if the room is not a sealed room, then Shizuma can be the murderer.

But it looks like a sealed room.

Thats what it mean to not judge a book by its cover. To sum it up, after that, Shizuma killed Yoshiko, and end his own life. Q.E.D.
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That means
Only Yoshiko can kill Yuriko.

That seems to be the case.

Obabas death should be the poisoning from the meal. Yoshiko probably poisons it while she murdered Yuriko. . Obaba did said I happen to passed by and notice the meal is already prepared. Q.E.D.
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(This reasoning lack the dumbwaiter clues and we didnt found Nene)

First is Obabas death at 1800
Where there a problem? Who could have hid the body? To be honest, I dont have the answer to these questions.

Ha? Then what is the conclusion of your reasoning?

The conclusion is I dont know. However, theres one possibility. A possibility that someone might have did something

Who?

The culprit is
Nene

Is that right
Why would you say that? Just because we didnt see Nene in the time re-occurrence, you can accuse her as the culprit!? What kind of conclusion is this! Do a better job on your reasoning!

Why would you say that? This is the conclusion I gotten from my reasoning. Now, begin time tuning!