You are taking the example when dithering is done independently for each color channel. Dithering can also be done across color channels, which can be useful for displays with more than three primary colors. Even in those cases, I found dithering to work better in the physical space, and not in perceptual space. I am trying to understand why.

Further, the question still remains why is it that mixing of photons spatially as you explained works better imperceptible pixels, and yet we need these non-linear color spaces when having larger areas.

Goes without saying that the intensity hit for 50 need not be the midpoint of that hit for 0 and 100 given the gamma curve, and actual mapping of the value to intensity for the pixel.