I haven’t seen any distinction one way or the other, though I may have missed it. But, some worlds/games exist in a plane of existence that is just flat. (Tabletop; Exalted, Video Game; Minecraft, etc.)
No — Midworld is round but its continents are packed together as a pangaea-style super-continent in one hemisphere of the world.
It is suspected that Dawnholme, the Empire of the Sun Elves, is on a continent or island on the other side of the planet. Some believe it is on a different plane of existence, which explains why they are able to travel so quickly across Midworld using portals. A small minority believe that they actually live on the Sun.
"The End to War, the End to Pain,
these will come in Seventh's reign"