I had another flash of inspiration for working out interim points! It would be far easier if you did this:
- Take your old point and your new point and make them into a 2 point polyline
- Densify the line to the number of interim points you want. See GeometryEngine.desify
- Crack open the geometry to extract the points which make up the densified polyline
- Move the graphic to each interim point for a very short period of time (you'll need to experiment with this)
The above method would work with any spatial reference. Not such an exciting solution from a software engineering point of view, but much easier.