Vanamonde, I regret that I cannot follow the substance of your latest message. I'll explain the problem I'm having with your first few sentences. Beyond that point, I'm totally lost. I'll state each of your sentences, end it with two hyphens and a colon (--, then start my response on a new line.

Center, inside, outside - These are ideas of four dimensions, one of time and three of the physical world--:
The dimensionality of a space is measured by the number of mutually perpendicular straight lines that it can contain at every point. Our familiar space contains three dimensions. Each can be described in terms of the direction you are facing. Thus, there are backward/forward, left/right, and up/down. The surface of the earth has just two dimensions, for example, north/south and east/west Special relativity recognizes these three spatial dimensions and the one dimension of time: future/past. comprising a total of four dimensions, collectively called spacetime. General relativity also embraces these four dimensions but adds at least one additional spatial dimension to accommodate space curvature caused by the presence of mass. The four-dimensional space that I envisage as containing our Universe is a four-dimensional hypersphere. It takes a good imagination to envisage this space. It has a three-dimensional closed "surface" analogous to the two-dimensional closed surface of a sphere, and it has a center in a direction perpendicular to all three mutually perpendicular straight lines that define the three-dimensional space comprising its "surface". It cannot be readily visualized.

For example, if we could have flown around in the universe when it was just 100 miles across, we would not find a center. It is just every time we moved 100 miles, we would be back to where we started--:
I'm not visualizing the geometry that you're trying to describe. This universe is 100 miles across, which sounds as if it's a circle with a diameter of 100 miles. What you described can't happen with a disk 100 miles across.

Pick a center, any center - we are ALWAYS at the center. But finding The Edge is like "chasing rainbows" --:
At this point, I'm lost. I hope you'll try again.