Thank you Michael.

I have not located a script which adds geometry, then does user picks, with Cancel not causing added geometry to remain.
I used .clone to reveal the top and bottom edges of the frustum, rather than a factory...
So far attempts to copyFactory the edges have not worked. It should be fairly easy :-) (I'll try again tonight.)
Removing the Cancel button from the display prevented it from being pushed, and leaving added geometry curves,
but maybe that is not a good solution if faulty geometry picks causes some hangup?

I see that Cancel of the BlendCap script will leave the selected edge, split into two parts. (They can be easily Merged back together.)

On another user picking topic regarding cone Base, or frustum Top and Base, picking:

For a cone, if the 2D Base curve is not a Closed, only part of the cone will be unwrapped to the Net.

For a frustum, the Top and Base curves must "Match."
Either both curves should be closed, otherwise both curves should end in the same line segments from Top to Bottom.

If the user selects the "wrong" Base curve, SlantSeam curve, or Top curve, or in the "wrong" order, the "wrong" unwrap (Net) will be displayed.
User verification that the Net is "correct" may be important.
- Brian