Isn't it the case that by invoking this red circle with center x, to use the shell theorem you must be physically at the edge of the universe for it to work? Because if this is not the case, you arbitrarily chose a circle and point x on the left side of your spaceship, which you could have equally as well have drawn to the right side of you in this homogenous universe. And those cancel out, similar to the trivial proof of Newton in corollary 2 (proposition 70). For his proof uses actual spheres, with actual centers; not arbitrary ones.

It is the case that by invoking this red circle with center x, to use the shell theorem you must be physically at the edge of the universe for it to work. Because if this is not the case, you arbitrarily chose a circle and point x on the left side of your spaceship, which you could have equally as well have drawn to the right side of you in this homogenous universe. And those cancel out, similar to the trivial proof of Newton in corollary 2 (proposition 70). For his proof uses actual spheres, with actual centers; not arbitrary ones. That is the problem with your scenario.