What is Book of Hymns?
Only Chapters (and Part) Should use H tags
Code:
<h3 class="chapno">Chapter 23</h3>

I wonder if the confusion is in terms
Headers and
Footers appear on all (print) pages
A '
Heading' (H#) is a Style of text. It just so happens that Calibre and Sigil use those as markers to generate the TOC with a couple of clicks. In addition, these are normal places where a new chapter starts, so the cod also get page-break-before:always inserted.
Stupid Vellum also codes title pages with h1 title, subtitle , h2 for series, author1, author2 ... (Styling like that was so HTML 3

)
Calibre convert turns those into individual screens (pages). Sinc I do WANT that to happen for all others, I do not turn that off. I just hand fix (chang all those OTHER h's to p on the title page