Edit Points tool from the 2D tools won't appear

The Rectangle and Circle shapes tools were changed in Motion 5.2 (maybe 5.1). (They're so much better now!) However, if you want to edit the individual points, you now need to Convert to Points (Object menu - or Command-Option-B). You can also Convert to Points by clicking the button in the Geometry tab of the shape's inspector.

You do see Modifier un rectangle in the popup menu list, that's actually a new command as well. The new method of editing deals with Rectangles and Circles as complete entities. The Square shape now relates to the shape in terms of Size (width/height) and Roundness. This means that the roundness of rectangles will maintain is "aspect" no matter how you change the shape. Previously, when you changed the shape of rounded rectangles, the rounded corners would distort as the ratio of width to height changed. Circles (ellipses actually) are now related as radii X and Y.

Both shape types have new onscreen controls: there are Midpoint controls to change width/height or radius x/y and an extra circle to control the roundness (or "curvature" control for ellipses) that appears to the left of the upper midpoint along the shape between the topLeft corner and the midpoint.

One strange aspect: it is impossible to go from a perfect Square to a perfect Circle with the Rectangle tool, but it IS possible to go from a perfect circle to a perfect square with the circle tool!