Great job on writing such an extensive volume on Cheetah3d. It's very well done and I'm enjoying it.
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Also you mention concerning the editing of the spline: "each control point acts like a 3D point and needs to clicked once to select it and then moved using the transform tool — you can’t just click and drag as in dedicated 2D drawing programs."
I was able to use tweak mode "Option + shift" to drag the points around interactively.
C3D v6.2.1 - more on splines:
For those used to editing vectors, splines, bezier points in other programs, the TWEAK mode works for adjusting 'handles' independently by using three keys "Shift-opt-cmd".
The sequence is to
(1)select.editable.spline.object (2)chose.Transform.or.Move.tool
(3)press.the.three.keys (4)click-drag.a.point.or.a.handle
The exception is the case of the user wanting to keep the two handles locked at the same angle. This case means lifting the 'cmd' key, holding only the 'Shift-opt' keys to adjust them.
Note that the right-click menu does not function with the "Opt" key pressed.
If you are already have a point or handle selected and see the widget, then click in an open area or Select None using cmd-D. Now use "Shift-opt" or "Shift-opt-cmd" to move points/handles.
Extra tips: Scale and Rotate (single spline points) and (groups of spline points) to make smooth shape adjustments.
Single points: the handle configuration is adjusted by Scale & Rotate
Groups of points: the points configuration is adjusted by Scale & Rotate
Scale and Rotate only make sense when points are selected, since they don't work well or intuitively in Tweak mode.
And don't forget the trick of using Tools:Set Value for the selected points in a spline for re-alignment or repositioning. Using Set Value for handles does not work.