I've been reading a lot about the twin paradox recently. I understand that there must be some kind of asymmetry between the twins' experiences in order for them to measure different amounts of time, and the asymmetry comes from the fact that one twin accelerates while turning around and heading back towards Earth. However, this is only an asymmetry if the twin in the rocket can't just as easily say that they're stationary and the twin on Earth is accelerating. As I understand it, the rocket twin can't do this because they are experiencing proper acceleration - something they can measure - whereas the twin on Earth would be undergoing only coordinate acceleration, not proper acceleration, from the travelling twin's frame. Another way of putting it is that the twin on Earth is in an inertial frame, while the other twin is in a non-inertial frame, so they can't use the same rules to calculate time dilation for the earthbound twin.
I have one issue with this though. What if the travelling twin's rocket and everything in it were accelerated by a force that acted equally on all points within every object. There would be no relative acceleration between parts of the ship, so the twin couldn't detect any acceleration, right? For instance, if they let go of a ball in the ship, it wouldn't move towards the back wall because the force would be accelerating all points within it along with the ship. If there is no experiment an observer can do to tell whether they're stationary or if all points in their body, and in everything at rest relative to them, are being accelerated by the same force, isn't proper acceleration meaningless? The rocket twin could now claim to be stationary and say the other twin was accelerating in the exact same way, with a force acting on every point in that twin's body. The situation between the two twins would seem to be totally symmetrical. How can this issue be resolved?