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And if the universe is spherical?

I think the question you are asking is what if the universe is toroidal. What if you traveled so far you came back around.

You come back around on a sphere too, e.g. traveling by the equator.

In that case you would be traveling a curved path so there would be acceleration even if your speed remained constant.

Isnt a torus curved? What's the argument?

If you use toroidal boundary conditions then you could remain in an inertial reference frame and return back to the same pont.