But when I posted my question, I was not contemplating legacy MOBI6 files with missing, incomplete or crippled metadata TOCs. I do not work with such books.
I own several Kindle books and not one of them lacks a complete, fully functioning NCX.
In fact, Amazon requires it in order to provide the next/previous chapter skip capability on Kindles.
Most Kindle books being sold today DO have an NCX. Otherwise, there would be no reason for Amazon to sell Kindle reading devices that GO TO directly to the NCX View instead of the HTML TOC, and there would be no reason for Amazon to issue a Kindle Previewer with a button to display the NCX View and another button to display the HTML TOC.