I mean, it seems most likely that the Meta-World is its own thing, like the upper-most layer of the story (the closest to us and Ryukishi), and exists so there's an actual consistent STORYLINE. I'm just giving my explanation for the interpretation I gave earlier. I'm not asserting it as truth, just a possibility for fun.
I don't see how it's unreasonable that Hachijo Tohya could write stories about fictional 1998's. EP3 could be a forgery, EP3's 1998 is just a fictional extrapolation based around the "real 1998".
Furthermore, the META stuff in each of them is connected, but need not have any real basis in reality. The fictional meta stuff is a part of the series that Hachijo Tohya constructed as "the means to illustrate the literary or thematic parts of the story."
Though about Meta-Ange, well, you have a point. Maybe Ikuko and Tohya had an author-battle about what they wanted the story to be about? Ikuko wanted to focus on Yasu, Tohya wanted to focus around Ange's struggle, and all of Umineko is Hachijo Tohya fighting about what the purpose of the story is
(joking)
Anyway, making Ikuko = Yasu just seems like something Ryukishi would do. Not that I'm really a fan of that idea.
edit: whoa, think I got lost in my own logic, nevermind >_>