EntireWorld

Does this include both sides of the RealityCheckpoint?

: Surely at least one side of the RealityCheckpoint must be outside the EntireWorld?

Certainly not (to both) - the world is smaller than the KnownUniverse.

How does this show that all sides of the RealityCheckpoint are inside the EntireWorld?

: It doesn't. It shows the opposite. The RealityCheckpoint may at the edge of the EntireWorld (it's certainly the border of the RealWorld), with only one side in the EntireWorld, although all sides must be in the KnownUniverse...