> I see one problem: Once one has some default state configured for Diff
> mode, it applies to patch embedded in Gnus Article buffers, but there
> the outline-minor-mode keymap isn't active and there's no way to toggle
> headings visibility... No idea how to improve this situation...

Oh, I see now it's a real problem.  Some patches are not displayed
because their hunks are hidden.  Fro example, a patch from bug#53770
attached below, becomes hidden with these settings:

  (add-hook 'diff-mode-hook
            (lambda ()
              (setq-local outline-default-state 2)
              (outline-minor-mode 1)))

For some unknown reason, outline--show-headings-up-to-level
first hides all sublevels, but never shows them again,
because or some level problem - it uses the level 21.