I am happy to report this issue as solved! : ^ )
After all my fussing and tweaking and ‘brute force’ attempts to wrangle Motion into working as I saw fit, the solution was to interpolate all affected keyframes as ‘bezier’ (in the keyframe editor)… and everything literally fell into place! Apparently the default keyframe interpolation for ‘scale’ is linear, and ‘position’ is bezier… and they caused animation that simply would not line up. Props are due to those kind folks that helped me out… thanks!
DJanzen