[Doitsu]

Originally Posted by helens

Incidentally, there's nothing I can do about it now (and it's not a major issue), but when reading the file on my (older) Kindle, the Table of Contents is greyed out.

On what Kindle model is the toc greyed out?
Did you convert the .epub file with Calibre to MOBI, AZW3 or KFX or did you use Send to Kindle?

Originally Posted by helens

Does anyone know which command (and on which page) would have enabled the toc in the navigation menu?

Depending on the target format, EPUB converters will usually look for the HTML TOC in epub3 books in two places:

The landmarks section in the NAV document (toc.xhtml):

  <nav epub:type="landmarks" id="landmarks" hidden="">
    <h2>Landmarks</h2>
    <ol>
      <li>
        <a epub:type="toc" href="toc.xhtml">Table of Contents</a>
      </li>
    </ol>
  </nav>

or the guide section in the .opf file:

  <guide>
    <reference type="toc" title="Table of Contents" href="toc.xhtml"/>
  </guide>

Please post both sections.

Originally Posted by Quoth

The Kindle Preview is not very useful for compatibility.

However, since Kindle Previewer is good enough for testing whether the toc menu option will be greyed out on Kindle devices, the OP might want to download and install it.