It's been a long time since I've dealt with 3D hardware so I may be just out of date. I know in windows it used to be forbidden to have two OpenGL contexts on top of each other. Having two contexts on the same page for sure is fine. The resources shouldn't be mixed up, they should be completely separated (I know in NativeGL there is a way to share across contexts/threads, but WebGL is different so i'm unsure).
I'm unsure how you are going to make one 'transparent', but curious if you get this to work. Generally the render target is a separate memory buffer, so having it be 'transparent' doesn't really mean anything. The output is usually a flip or a copy to the target area, flips imply the memory isn't actually moving at all but the HW that updates the RAMDAC has specialized code that says 'hey, at buffer address X actually copy the memory from Y instead'. You would have to render to a target, then copy that target onto the final target. Otherwise you'd just be transparent onto some random garbage or whats already been drawn. Maybe this is implicitly done somehow by WebGL?
The PolyLineCollection will recreate itself on its ModelMatrix change, I think it does this due to 2D/Columbus mode in which it needs to re-project itself. I guess its semi-unavoidable in cases like that. But in the 3D mode there is no need to recreate or project anything.
A bigger issue i'm having right now is needing to update the path, adding a point about every 100ms or so (becoming circular at some threshold). Ideally this would only project only the 'new' point since the others have not changed, but the code is structured such that every point is re-projected, for static views that's fine, but for updating/dynamic content its a bit much. Considering that Cesium has CZML for dynamic/live views I would have thought the base code was a bit more prepared for this.
It performs ok, but its chewing up the entire CPU core (even at a forced 20fps instead of 60fps). And there are other things that on the page that need some of that good old CPU time