If I understand you correctly it looks like your normal calculation is throwing off the orientation of the plane. Look at the normal calculation in this example. Is this the desired goal?
Not quite. It looks like the plane is orthogonal to the line drawn. I’m using the line to define the plane, so ideally it would be drawn parallel to the line and with a width equal to the length of the line (hopefully that makes sense).
I’ve been using the Plane to do calculations, so I think it’s defined correctly. I think that Cesium is reorienting it somehow (maybe using NED) when drawing it.
I have previously created an example of drawing a polygon through 3 points, which you can check out here.
You can test it out by zooming in and left-click to create 3 points (or more) on the terrain, then right-click to terminate drawing and generate the polygon.