"Just personally, I've never seen a truly convincing mechanic for explaining just how the last one would work."
Go read about the poincare disk and non-euclidean geometries... spherically-shaped universes which work similarly to general relativity (curvature of space-time, think about that statement) which means that the closer to the edge you get, the harder it is to get there, similar to the faster you go the harder it to accelerate. Basically, the edge of the universe is a limit in the mathematical sense, as well as the physical one, such that as you approach it, it becomes increasingly harder to hit it. This means that your distance relative to a certain point distant will approach a constant value, isn't that the definition of a wall? (even if you cant see it)