Finally managed to get webmentions working on this site, to my great satisfaction. There are, of course, plenty of things left to sort out. One relates to formatting, or presentation. Some of that I can control. Some I cannot, as it depends on what the person at the other end sends. A, for example, appears twice, because that's how it is marked up on Daniel's site. Not a lot I can do about that, although I will note that it was one reason why I originally wanted to mess with the Grav plugin to make it use a different, simpler, parsing engine for microformats. And some webmentions that ought to have appeared have not, yet. That's something else to explore. I also need to add the little script that allows people to make use of sending a webmention even if they don't send them out automatically.
Beyond that, there is a kind of philosophical dilemma, not unlike the original dilemma of owning your own content in the first place. I am utterly dependent on the good offices of other people to maintain the services on which I now depend. Still, at least they're not evil mega-corporations.