Re: gray. Yes. I didn't say it was a medium gray, I said it was a neutral gray (neither warm nor cool). But you're absolutely right, this model is merely an approximation of perceptual color. You could instead build a complicated manifold representing perceptual color space, and define a criterion for a cloud of points in that manifold that resulted in pleasing colors, but it would be a lot more work than using the sphere, and you'd not gain that much, I don't think.

Re: how do we know? Well, mostly this system is based on the work of a number of thinkers, starting with Goethe and running through Munsell, Itten, Kandinsky, and a bunch of others, who spent a great deal of time putting colored squares next to each other. They discovered physiological effects of color, like simultaneous contrast. They attempted to learn about psychological effects of color. They admitted that, while some color combinations are attractive only to individuals, there were other sets that seem to have universal appeal. One of the things that those universal combinations had in common was that they centered themselves on the color wheel.

We know, more or less, by science. It is a largely empirical question (put some colors together, see if people like it). You can repeat the experiments yourself, if you like --- you asked 'how can we know'? Either take their word, or try it yourself.

And finally, in reply to the my use of the word 'emotional' --- if I were to say that colors whose center of balance is far from the center axis of the sphere are more surprising, and less comfortable, is that acceptable? I don't mean to say that red and green don't evoke different emotions than yellow and purple --- but they are both comfortable, obvious, familiar combinations; while, say, magenta and brown is less comfortable and takes more work to accept.