Two last tests:

Using 64 points, _TrimCurve yields 61 segments. (3 are of double length, unsplit)

For standalone open curve, if the 61 segments are then Joined, and the _TrimCurve repeated, 64 segments result, the correct number.
(For an open edge, using Merge on the 61 segments, and repeating _TrimCurve with 64, results in 64 segments.)

Second tests, with stock Trim command, with start point added, yielded 1023 points versus 1024, one less than correct number of segments.
I'll try adding start point to _TrimCurve test script...

- Brian