> Have you found a way to combine char-folding with case-folding?

Yes. This particular patch uses regexps, so it works independently of
case-fold-search. That is, if case-fold-search is on, then you will get
case folding in addition to this.

> (in https://lists.gnu.org/archive/html/emacs-devel/2015-02/msg00407.html
> another hard problem was char-folding with a combining character.)

That works too. "a" matches both "á" (the letter) and "á" (a letter with a
combining character).