Touché. I should clarify that ox-hugo touches the static/ dir only to copy attachments (images, pdfs, etc.).
Enabling copying/overwriting of CSS would fall too much out of ox-hugo scope.
ox-hugo is only for content management… which includes the images associated with your site content (markdown).
Hope that makes the scope of ox-hugo clearer.
Correct. But they are useless unless you update your layout files to use those CSS files. So you anyways need to edit the layout files manually. So you might as well copy the CSS files manually.
Of course it is possible! But you need to do that in your layout files. In my bare min theme, I inline the CSS, but you can put in that link tag if you like.
Questions for ox-org would go to Org mailing list: emacs-orgmode@gnu.org
That has nothing to do with ox-hugo or Hugo 
You’re welcome.