>> 1. Should the recursion stop when ‘fixed-output-derivation?’ returns #t?

> I think so.  In general fixed-output derivations are the things you
> would like to pre-fetch; so you would typically call ‘build-derivations’
> on these to do the actual pre-fetching.

Could you show a package with a fixed-output derivation?  My current
implementation calls itself indefinitely because the base case is never
satisfied.