The acceleration isn't constant in the scenario you're describing in #13. It can be directed "towards the floor" the whole time, but only if the rocket is rotating, and if it is the rotation is detectable.
The equivalence principle says that there are no local experiments you can perform to distinguish between constant proper acceleration and a constant gravitational field, but the word "local" refers to the fact that the equivalence is only exact in the limit where the "diameter" of the region of spacetime where the experiment is performed goes to zero. The final ages of the two twins depend on the curves in spacetime that represent their motion. There's nothing "local" about that.
