> - add a function to the Guix ".dir-locals.el" (does dir-locals support
>   functions at all?)

I haven't tested, but currently .dir-locals.el contains

--8<---------------cut here---------------start------------->8---
 (scheme-mode
  .
  (indent-tabs-mode . nil)
--8<---------------cut here---------------end--------------->8---

I suppose we can add

--8<---------------cut here---------------start------------->8---
 (scheme-mode
  .
  (indent-tabs-mode . nil)
  (guix-devel-mode . t)
--8<---------------cut here---------------end--------------->8---

> - the function to set up a third-party package (Emacs-Guix)!  Why Guix
>   should do it?

Sorry, what do you mean?

-- 
Pierre Neidhardt
https://ambrevar.xyz/