Well, the book is posted (not yet available, so no link!). Thanks again to everyone who took the time to reply to my queries. In the end, I just forgot about the GIFs and replaced them with JPEGs. The rest of the formatting looks great (well, it's what I wanted anyway
), and the TOC now works on both Kindle devices and Android tablets. I have to assume the other device categories are likely to work too.
Calibre's TOC processing actually did work for my eReader, but I needed to make that edit to the OPF and HTML to get Android Kindle to enable the Contents page.
Next up -- ePub (well, in a few days, maybe).