Madness is like exam results; the border is defined by social convention
such that just the 'right' people are sane.
And mimsy were the borogroves. Reality is what is.....sanity is measured by an individual's grasp of reality.

Reality defined is what is real, regardless of whether something is known or not. Not knowing (thinking) something doesn't change anything except as pertains to the individual who is ignorant of whatever facet of reality they haven't discovered. Proof in point is mathematics, or better yet for this list, fractals. Fractals always were, but the discovery had to await the developement of computers. Fractals exist, are real and a part of reality. If some bushman in the Kalahari Desert doesn't know about them then his knowledge of/about reality is limited, but this doesn't negate anything, it only places limitations.

I don't think so. Insanity is a measure against an individuals ability to "see" reality. Since we don't know everything reality (understanding) is constantly expanding and unfortunately there are some retards who can't accept this, but acceptance is not the basis for reality. The insane are mentally color-blind. An individual that can not see color(s) or hue(s) is defective......no other way to put it. Still, even so, color exists, and independent of the biological eye, too. IR and UV require special devices to be visible to the human eye, but nontheless are part of the spectrum at opposite ends.