Hi Nate, The c1, c2 confused me too. They relate to the velocity curve the speed at which movement takes place, not any physical movement in space. They are just two values not an (x,y,) pair. In your example moving from 0 to 0 can take place at any speed profile since it doesn't move at all. I do appreciate your point though that for actual movements, getting the velocities to join up at each end is tricky if the amount moved is different between different key frames.
As far as I can tell you can only move a sprite position in a linear manner between two keyframes. (Or change a rotation relative to something else in a linear manner.) There are no bezier controlled movements between keyframes in Spriter.