I am not going to bother citing articles, interviews, the manga or what have you, as I am essentially a French Formalist: What you see is what you get. Instead, I will try my hardest to give you an adequate explanation of what happened.
You are correct, in a sense. People did stop thinking, and the interpretation that it is because of some insanity regarding different Game Masters in the meta-world is nonsensical, however, it is still very likely that Yasu is the culprit because of what is said of the culprit and her means in Our Confession. If you view that Our Confession is as much a part of Umineko as Game 1 or 8, as I do, a view which I will now explain, things should become more clear.
Here's an assumption for you: Umineko is a deconstruction of the mystery genre. More specifically, it is a deconstruction of the Golden Age of Mystery, which features ideas such as the detective proclaiming "A-ha! I have solved the mystery." Reader beware, you're in for a spoiler if you turn that page... and this is a feature of the mystery genre which is mentioned several times throughout Umineko. While we are left without a traditional "answer" in the main bulk of the game, Our Confession provides us an answer that works in every single game board: Yasu is the culprit, and she has the resources to hire accomplices.
I now pose you the following idea: What if Ryukishi decided that while the answer is best left unread (a warning given at the start of Our Confession,) given as this is a mystery, and not a fantasy, he was obligated to provide an answer anyway? Would he not take all possible precautions to separate this answer from the rest of the games? You have to go looking for Our Confession, it isn't just there for you to read, within the bookbinding of the rest of the game. In that sense, Umineko even deconstructs this idea of turning the page and reading the "answer" to solve the culprit.
To cut to the chase, I think the fact that Our Confession is deliberately segregated from t
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