First, if you only want the replies to go to one email address, you will
not use the &Cc=$FROM:U$ part. Again, if I take what you said strictly
you don't want to bother with an In-Reply-To: header and you can leave
that paramter off, too.
So I interpret your plan to involve two different kinds of URLs:
You need to make each of these URLs the HREF attribute of an anchor in
you message-page resources, such as BOTLINKS.
for how I did it.
Wrap the anchor in text that tells the user what they do if they
exercise this link.