I've finally figured out how to make the Mooquackwhatnotbot include next- and previous-entry links in each entry's page. The solution involves defining an XSL variable at the point where the link appears, and referring to the previous/next entry using the current entry as the axis; evidently I didn't think of this the first time round.
I included the “Find this entry in the archive by date” link as a substitute for next and previous links when I found the latter too tricky; it's staying because: it provides a nice chronological overview of the entry; the archive is primarily ordered chronologically; and it's doing no harm.
Also, because the solution uses <xsl:if>s anyway, I get the problem of cockups on the first and last entry fixed for free.
I originally planned to put the next and previous links at the bottom of the article, in the main content area. Looking at it now I think it makes more sense for them to go in the Blurb, with the links to the archive, thus leaving the content area for the content.
In loosely-related news, the address of the Atom feed is now tidier – I already had a PURL at http://purl.org/mooquackwooftweetmeow/atom pointing to http://purl.org/mooquackwooftweetmeow/atom.xml, which itself uses a PURL. So there was absolutely no reason not to drop the .xml to match the entries' new URIs.