thanks for the replies.
The additivity issue is what I've been trying to wrap my head around.
Barring color decoding artifacts, what would be an example of a failure of additivity in any given display?
Let's take the first question
You have the correct primaries, so your basic units which you're adding together are sound.
You have your white balance perfect across all 256 luminance steps, which means that the correct amount of signal is mixed together for each of the three sub pixels when the display renders shades of gray.
I'm assuming that saturation (or more accurately, excitation purity) would be reflected by the amount of white that is added to each primary. So if you're adding the correct amount to the correct primary, how come the saturation sweeps are not guaranteed to be accurate? To make it simple, let's assume that the saturation sweep also sweeps upwards in luminance, so that you start at red, and work your way up to pure white.