Orientation in 2D

[_Aka_]

Hopefully that's more clear.

Absolutely. Thanks for the details.

The shape/complexity of the spline in the middle does not seem to matter at all.

When using dynamic orientation, the orientation is deduced from the Orientation Anchors rotations (in addition to swirling). By default, the first and last CPs are Orientation Anchors. Then users can choose to set specific CPs as Orientation Anchor.

getOrthoUp0INTERNAL() uses a static Vector3.up reference which may not be appropriate for 2D but I wasn't able to change that in a way that yielded better results

The Vector3.up is multiplied by the CPs rotation, so chaging the rotation should do the trick, or am I missing something?

Auto End Tangents toggling on and off didn't seem to have an impact

This setting is irrelevant to your current concerns. This setting is used by spline interpolation modes that need at least 4 CPs to work. It is used to decide if the spline should "duplicate" behind the scenes the first and last CP to get the sufficient number of CPs.

The end of ProcessDirtyCheckpoints for !closed splines does something but I couldn't figure that one out

If you mean this code

Code:

// Handle very last CP
if (!Closed)
{
  CurvySplineSegment beforLastVisibleCp = GetPreviousControlPoint(LastVisibleControlPoint);
  LastVisibleControlPoint.ApproximationUp[0] = beforLastVisibleCp.ApproximationUp[beforLastVisibleCp.CacheSize];
}

This gets the last up of the last segment, and set it as the first (and unique) up of the last CP (wich is not a segment, since the spline is open).
About the CPs visibility, the default behavior (splines having Auto End Tangents to true) yield splines with all their control points visible.