2. You could use a build step for this, yes. Personally, I don't because the theme files more or less contain always the same tokens. I rarely add any (colors being the most frequent) and if I do, the cost of it being unused is minimal. My standard full theme file is around 5kb.
3. Stylelint complements my work but isn't necessary. My code does not depend on it to work, contrary to Tailwind, for instance. I'm free to stop using it any time.
4. It's a monolothic file containing all the code needed for the whole site and not only the page I'm looking at now. It may not make that much of a difference in performance, but still, the browser downloads CSS code it does not need now and then must read through all of it to apply needed rules. Not optimal.
By the way, it's great you came back with further questions! Hope to have addressed them at your satisfaction. Still at your disposal if you have more!