FWIW, Kawa has define-alias which is more-or-less the same thing:

https://www.gnu.org/software/kawa/Locations.html#idm45230717976176

Most of your examples work with no problems on stock Kawa
if I replace alias by define-alias.

For the record, the ones that don't work are:

(1)
    (let ((y +))
       (define-alias x y)
       (define y *)
         (free-identifier=? #'x #'y))   ⟹ #f rather than error

This could be an extension because of more liberal Kawa ordering rules;
I haven't looked into it.

(2)
  (alias inject unquote)
     `(list (inject (+ 1 2)) 4)
==> (list (inject (+ 1 2)) 4) rather than (list 3 4)

This may be just a missing followAliases call in the implementation.

(3)
    (syntax-case #'pear ()
       (pvar
        (let* ()
          (alias fruit pvar)
          (syntax->datum #'(a fruit)))))
/tmp/x.scm:34:30: reference to pattern variable pvar outside syntax template
/tmp/x.scm:34:10: invalid argument to location

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	--Per Bothner
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