As far as I understood:

1. All the columns are disabled initially.
2. The first click on a column enables its descending order, the second - ascending.
3. You can't disable all sortings after you've enabled any without reloading the page.
4. Sortings doesn't mix with each other.
5. Sortings never separate dirs and files.

Feature requests:

1. How can I get my initial sorting order back without reloading the page?
2. It's not really obvious that the sortings don't mix. Maybe, show it with icons, as it is in the most file managers?
3. What about additional setting for default preferred sorting?
4. Directories and files separation?

At present, all the sorting is just the browser default; the script does not do any sorting on its own. Different browsers handle this in different ways. I note that the Chrome (Vivaldi) default is to put dirs at top, but Firefox just alphabetizes everything. There could be OS differences, too. A preferred sort setting should be possible however, but I'm not sure I want to dive into adding other sort functions to the script just yet. Perhaps in a later version.

BTW, thanks for all your great input!