The issue is now resolved.
very much for the AZW3 hint, eschwartz, which set me off in the right direction. Here is how it can be accomplished:
- Convert your HTML file into AZW3 in Calibre.
- Using Calibre's editor, move the ToC section to wherever you would like to have it in your e-book, then save the AZW3 file.
- Perform an AZW3 to MOBI conversion in Calibre. Somewhat illogically, check the Do not add Table of Contents to book option on the MOBI Output settings screen.
- Voilŕ! The MOBI file has retained the ToC location from the AZW3 file without adding any additonal ToC screen to the beginning or ending of the MOBI file – and the ToC menu button is functional as well.