For the atmosphere, what if you got rid of the Planet Atmosphere and used a solid cloud layer (unplugged Density Fractal) instead, with a depth that matches the height of the ringworld section (not to the local roof but the entire section). Then localise it using a side profile mask that matches the upward curving profile of the ringworld, and graduates out at the top (projected as a mask from the side). Also merge an additional mask to soften the sides, but that one projected from the front.
Just a thought. Really nice work so far.