Chris Aldrich is way too kind, but it is the kind of kindness I need. A couple of quick responses:
I will indeed tweak the regex as you suggest. That behaviour by reading.am does rather spoil using it as a repository for links I want to go back to, but hey ho. On posting to Wayback Machine, I thought I saw a blog post from Reading that this was now done automatically for all saved posts. Getting things into my Known stream is on the horizon, but I will have to learn a lot more about how to use the web publish tools.
The webmention form is now looking good and working well, and I think I may even have got email notifications up and running, thanks once again to great help from the IndieWeb community. I'm not happy submitting anything to the original plugin, because it is so very different in scope and approach. But I am definitely thinking about creating a plugin of my own, which would also make it very easy to send the notification email. That's definitely on the cards. Putting it all on the wiki is next on my list.
And thanks again for being a guide and an encouragement.