Aa for the questions of the UNIX-ness of X, it started in Athena, which as I understand it was supposed to be relatively OS-agnostic distributed computing? In any case, the predecessor ran on a different OS, not sure how significant that is to the genesis of what would be called X or what OS it "started" on.

Athena was about scaling up Unix workstations. It was started with grants from IBM and Digital. It was never OS-agnostic.

You can find a brief history of X at https://www.oreilly.com/library/view/x-power-tools/9780596101954/ch01.html, and the Wikipedia article on the X Window System is pretty good; reed the "History" section.