Nice questions. Thanks.
No, just like the surface of a sphere, my universe does not have a spatial ending. If you are talking about time, that is a different question entirely, and to attempt to explain it very quickly is impossible. In a word, the temporal extent of my universe is: 1. However, this 1 is not just any 1. As you can see, it is unit-less. We can discuss this idea more if you want.
When I talk about Einsteinian relativity, I am referring entirely to the notions of length contraction and time dilation. Again, my reasonings for my disagreements with relativists are extensive. I would just say that a critical reading of Relativity: The Special and General Theory
by Einstein reveals many logical "irregularities".
Every gravity field in my universe consists of a singularity. This is a point of unimaginable density and smallness. No gravity fields are permanent. They come and go with no rhyme or reason. You can think of gravity fields as the inhalations of God, if you wish.
I don't know anyone who says that the universe is "built up from atoms". Some say that atoms exist within the space of the universe. You have to think about energy, too. Regardless, pretty much everyone says that atoms have mass and therefore atoms have gravity fields of their own.
In my universe, however, gravity is an entirely cosmic force that creates matter (contains energy), destroys matter (liberates energy), and structures vast regions of the universe.