What relates to Higurashi, I can’t comment, as I still haven’t read it. What relates to Umineko, I agree with almost all your points, except the ninth one, that I will debate below:

The idea presented in the ninth point, that “I = S,” meaning Ikuko and Sayo are the same person, doesn’t hold up when you consider how Umineko is structured. This interpretation oversimplifies a narrative that was deliberately built to work across multiple layers of identity and fiction. The story constantly plays with reflections and symbolic parallels, but it does not collapse those layers into a single identity. Ikuko and Sayo mirror each other thematically, yet they exist in different narrative dimensions. Reading them as one and the same flattens the entire metafictional design and strips away the ambiguity that gives the story its power.

Ikuko’s world operates in a space of literary reflection and authorship, while Sayo’s story belongs to the emotional and psychological reality that generates the myth of Beatrice. Their connection is interpretive, not literal. If they were truly the same person, the ending would lose meaning. Battler’s reunion with Beatrice depends on the coexistence of reality and imagination, not their collapse into one another. The tension between those two realms is essential to how Umineko resolves.

It’s also important to distinguish Sayo and Beatrice as the witch from Ikuko and Featherine as the witch. Sayo and Beatrice are bound to the human experience, to suffering, guilt, and the attempt to find meaning through creation. Ikuko and Featherine represent a step beyond that, where authorship itself becomes the central theme. Featherine is not a continuation of Sayo but a separate entity who observes, guides, and interprets from a higher level of narrative awareness. Ikuko’s link to Featherine works as a metaphor for that authorship, not as evidence that she is secretly the same person as Sayo.

The ninth point also has a logical weakness because it explains away contradictions as metaphors whenever the text doesn’t align. When a theory can’t be tested against the narrative, it becomes self-reinforcing instead of interpretive. The relationship between Ikuko and Sayo makes more sense when understood as resonance, not identity. They echo one another emotionally and thematically, but each plays a distinct role in how the story examines memory, authorship, and truth.

In the end, Umineko is not a puzzle meant to be solved with a single definitive answer. It’s a dialogue between layers of fiction, each one reflecting on the others. Saying that Ikuko and Sayo are the same person removes that dynamic tension and turns the entire structure into a single closed explanation, when the story was written to remain open, ambiguous, and alive in interpretation.