As discussed on emacs-devel, here is the patch that implements
a standalone command that reads a list of files and replacement strings,
then shows a diff to review before applying replacements.
Also provided the Dired integration to show the replacement diff
on marked files. Later the same function could be used
to show replacement diffs from the xref buffer and maybe
from other packages as well.
Here's a counter-proposal: we were talking about a "refactoring" packages
on emacs-devel, maybe a week ago. And I suggested a function that would
take a list of changes (as some data) and present them using some
customizable logic: the current Eglot's solution uses a diff, and I'll add
an implementation that shows a tree-like buffer with checkmarks, probably.
I'll be starting on this any day now ;-(
So... provided this won't take too long, I would suggest your code here
just focuses on creating a list of changes (those shouldn't require buffers
to visit files), and then you'd be able to pass them on to
'refact-show-changes' (name under construction), which would then use the
interface that the user prefers.
This was we'll also consolidate the diff-generating code for features of
this sort.