Error in my .py screenshot. The first line should read:
if unreal.planeTrackEditableWrapper.call_clear_spline(actor)
You may infer from this and my C++ code that planeTrackEditableWrapper is the name of the class containing my blueprint interfacing functions, which is also the parent of my blueprint class.
Probably should have mentioned the class parenting part as that lets your C++ talk to the blueprint, let me know if you need to hear about that.