Although I haven't gone back to look at comment webmentions again (I'm going to let my thoughts on that ferment for a while) I did decide to make the replies to comments feature live. It's still only one layer deep but it's better than nothing and, at least, allows a bit of structure.

@colinwalker I saw this and thought “isn't he using WordPress?” I went to your Colophon and was immediately corrected on the matter! :)