Why is it so unbelievable that the Myth World is flat? Doesnt Tharsis have enough oddities surrounding it that it being the center of the world can seem believable? When one considers that one mediocre sorcerer could destroy the entire world from inside it, that it is arguably where Nyx forged the Trow (some might argue that this is just a human myth), and that it errupts only at the close of every 1000 year age, it certainly doesnt seem unlikely that it could be the center of the mythworld as well.
Your assertions seem a tad to definative for my tastes considering that there is as much evidenc to suggest that the myth world is flat than to suggest that the myth world is round: the "center of the world" theory (which I take literally) and the "mad journeyman's" rantings both point to the myth world being flat while the skys of the myth world seem to point to it being round.
-Milky