I made the FX assumption just to keep the calculations simple. It doesn't really change the underlying point. While resolution certainly affects image quality at some point it doesn't affect motion blur as long as there is enough pixel density to resolve the blur. My point was that in all cases of Nikon DSLRs given the shooting conditions we are discussing, there is plenty of pixel density to resolve the blur. Thus when we are analyzing the possibility of blur for these conditions we can completely remove the issue of sensor pixel density from the analysis for any extant Nikon DSLR -- DX or FX.
The only issues that matter for motion blur are: how fast is the shutter, how fast is the subject moving relative to the frame. The frame can be the whole sensor or, if you crop, the cropped frame.
Of course, you could crop down to the point where the number of pixels spanning the motion blur were so few that you wouldn't be able to distinguish the motion blur, but at the 1/500 shutter we are talking about your image at that point would be all but unusable due to lack of resolution anyway.