Thanks for the patience.
Thanks for the feed debug code. Today's feed soup contains:
So I can now see the feed Calibre is working on.
It does look like the offending text has a <p> tag following the </sup> tag (bolded). Also there are empty <p> tags (red).
I might try cleaning sequence
</sup><p>text</p> up to be
</sup>text
Also as you suggest, I'll try not using the embedded text.