I’m not an expert in this but know the relevant physics well. Here are my thoughts on this good question. Answer 1: Gravity is just like electrodynamics in thi

I’m not an expert in this but know the relevant physics well. Here are my thoughts on this good question. Answer 1: Gravity is just like electrodynamics in this respect: as you say, the force vector is determined by the position of the other mass at the so-called retarded time not the current time. Specifically toward the position the other mass would occupy right now, had it kept moving at constant velocity using the velocity its velocity at the retarded time. Hence, all deviations of the force vector from current position are due to the acceleration of the gravitating mass. The significance of the deviation is roughly estimated by the dimensionless ratio : a*x / v^2, where v is speed, a is acceleration, x is distance from gravitating mass. Accelerations are very very tiny in galactic contexts, so numerator is smaller than denominator. Answer 2: A galaxy is like a fluid with approximately steady flow; hence, even if a given gravitating mass has moved since the retarded time, some other mass has taken its place. I haven’t put numbers on these though.