This is true — I take the upshot of this observation to be that only proper times are ever directly physically measurable. Coordinate times and concepts that re

This is true — I take the upshot of this observation to be that only proper times are ever directly physically measurable. Coordinate times and concepts that rely on them (such as what you call one-way velocity of light) are always merely a bookkeeping device for organizing the results of different proper time measurements and will always entail some amount of arbitrariness. General relativity makes this more explicit: the arbitrariness is that of the choice of coordinates on spacetime, and one can choose any old coordinate system or none at all.