If you're going to say or imply that surviving these experiences, or being
    tolerated for experiencing them, was only "a small proportion" then you're
    making broad assertions.

It's a broad assertion I feel safe making from what I've read, and from what I know of human nature, and the often toxic dynamics of small communities. Being different in a small tribe sucks. Yeah, if you're different in just the right way, you're venerated as a shaman or religious figure. But I feel absolutely safe in asserting that for the average mentally ill person, being mentally ill in a pre-modern society would have been worse, both in quantity and quality of life.

Maybe you wouldn't have been exiled, as such. But you would have been a marginalized outsider in an environment that was far more resource constrained than the environment in which we live today, and which rewarded conformity to a far greater degree.