Originally posted by Ivan Seeking
We still have no preferred observers in the inertial frame.
I'm not sure what point you're trying to make.
If one accelerates, that frame is no longer valid under SR, and we can then make a distinction between the two systems.
However, acceleration is not necessary to distinguish between the two systems. As I said, the only thing that determines whether two twins age asymmetrically is whether the spacetime lengths of their worldlines are different. The symmetry can be broken by acceleration, or by other means.
We can resolve this using pure SR, by the way: my calculation was performed entirely in an SR inertial frame (that of the Earth twin).