Could you explain what precisely you would like to know and post some single frame images? Your youtube animations are heavily compressed so it’s even hard to guess what’s wrong with them.
You should always render out to frames, never to avi or mov files. It something goes wrong you have to re-render the whole animation again.
Render to tga’s, exr’s, png 48bits, etc
Bring that into your editing app or compositor and compress to what ever file format you prefer.
It can be done if you use lots of path tracing samples. 6 passes virtually means 6 subdivs per pixel. This should even produce nice antialiased edges in some cases. I usually try to use as little passes as it can be - why should anyone use for example 100 passes?