I see a short code snippet and two small images. Can you carve out the actual question and create a Sandcastle for that? Maybe this will make the intention clearer.
If there is a question about clipping planes, then this should probably be a separate question.
(The only thing that these questions seem to have in common is that there is some confusion about the coordinate systems, but it’s hard to make a more specific statement than that…)
I created a sandcastle (based on the ‘Sample Height from 3D Tiles’ Sandcastle) :
It takes the two points of the polyline, and creates a plane that is aligned with the plane that is created from these points. Comparing that to the first image that you posted, this could be roughly what you want to achieve:
(Aligning the plane and its size perfectly with the exact polyline is trickier than it looks at the first glance. But maybe the code helps to solve the original question about the “Normal vector for plane entity”, at least…)