Total layman jumping in here, but in the past I've wondered why the expanding space factor doesn't need to be included in calculating local mass-mass interactions. Even though the expansion is something exceedingly small (like 60 km/3 million light years every second or so?), it seemed that it should be included for precision in calculating how masses will move with respect to each other.

The typical answer (summarized) is that "local mass interaction totally overcomes spatial expansion, so only the gravitional effect exists in local systems", but it still seems that there would still have to be some accounting that some of the gravitional "pull" is having to be "used up" to counteract the expansion.

Your explanation above appears to make this even more necessary, since if we think of the expansion as negative curvature (which is in fact really the case), then even local space is, however minutely, curved in a negative way due to expansion. Therefore, any positive curvature of space is being exerted on that already negatively curved background, and hence the positive curvature of space would necessarily have to be minus whatever that negative curvature was (however miniscule).

Unless I should have been interpreting the typical answer to mean "local mass-mass interactions are of course affected by the expansion of space, but the local mass interaction is simply so large with respect to local spatial expansion, that the local effect of spatial expansion, while not zero, can be ignored for calculations". Or something to that effect.