[Dormin]

I believe you are pretty close to what ryu meant as "people believing to play a game". In that way you make lots of logical points. But I must disagree with few points:

First of all, the deciding of the next head by solving the epitah. Even if we have heavy implications that kinzo originally thought the headship to be decided this way, I have very hard time believing that any of the relatives would actually accept yasu as the new head. I know this was also heavily implicated to be possible in ep 2, but now we still must remember that yasu ultimately is very crazy and her crossdressing tricks don't necessarily make other ushiromiyas trust her very much. I don't know even if it is legal to actually decide about inheritance by some riddle. Also, if crossdressing teenager girl/boy suddenly appears in front of highly intellect individuals and tells that she is now the new head, how do you think the sisters would actually react? Would they accept her, or would they find some legal trick to have the gold out of her hands? They have no way of knowing that she is actually even related, maybe the word of genji, that also was seen very untrustable as a servant of kinzo from the pov of other relatives.

However you are right that the general layout of ep 4 was probably supposed to resemble this and I can see this happening, if we forget about overanalyzing things.

I can agree with other claims about accomplices, except these:

I can believe in an accomplice who goes along with Yasu, believing that nobody is in any true danger.

I believe this, theoretically, but I'm still not convinced this kind of scenario could actually happen as most of the people consider something shady going on when confronted by a madman bribing you with gold. Even though they wouldn't know she is actually a madman, the parts about having suddenly inheritance and tons of gold and every part about hiding this for a long period of time for some reason, at least in me, would raise a shitload of doubt.

I can believe in an accomplice who is actively evil.

Evil is a funny word and I don't believe even genji to be "evil" in a way he would be okay with massacre. This topic can be discussed, but I think this claim for an accomplice should be stated "accomplice who has a motive to be an accomplice". I can believe someone for example in the need for money to go along with plan as long as it is properly planned and there is mutual trust between accomplice and culprit. I think any claim that has yasu going "I have lots of gold, now kill for me!" is basically unbelievable as long there is no guarantee of actual wealth or a plan constructed properly with time.

There's a bomb on this island and I'm holding the remote right now.

Bomb is a very vague threat. Gun will probably work. But as probably stated before, yasu has no evidence of the existence of any explosive device. Now, we can discuss whether it is logical to be careful around bomb threat, and we cannot go past the fact that characters are indeed individuals who act to what they see is logical, but having a teenager present you a "possible bomb" would probably not be the most believable threat, at least not as believable as gun in your face.

However if combined the effect could be different. I don't know what I would do if conformed with a vague bomb threat, probably be very worried and panic, but thinking non-emotionally and logically, threat that has no evidence of existence is not very threatening