After thinking about it, I'm not convinced that you are right, but I am convinced that it's much more complicated than I was thinking it was. The problem is that, as you say, things like lengths and proper times and proper accelerations are assumed to be measurable in most SR type thought experiments, while they are no longer measurable if you assume a universal "gravitational force". The notion of proper time in the theory SR + gravity will not be the same as the GR notion of proper time, and similarly for length measurements and proper acceleration measurements. That makes the comparison of "SR + gravity" with experiment exceedingly difficult.For example, what does proper acceleration--path curvature of a worldline--mean in this "SR + gravity" theory? I know we've gone back and forth about what different types of accelerometers would read, but in standard SR and GR, a key element of the physical interpretation of the theory is that proper acceleration, path curvature of a worldline, is a direct physical observable; there is *some* device, call it an "ideal accelerometer", that measures it. A given worldline in the presence of gravity has *different* path curvature according to "SR + gravity" than it does according to GR, because "SR + gravity" still uses flat spacetime; so "SR + gravity" can't possibly get good agreement with the physical predictions of GR with regard to path curvature.