@nontroppo, thank you for your calm academic approach.
To address your final point first: I assume that Smashwords hasn’t updated the Meatgrinder because, well, it works, for a value of “works” that requires .doc file upload. If I wanted I could upload an epub to Smashwords instead, but then I wouldn’t have .mobi and PDF versions version for sale there as well, nor an HTML viewable preview (like an Amazon “look inside”.) I’d still get distributed to Apple Books, Kobo, Barnes & Noble, and several also-rans, though. That said, until my problems yesterday I was a happy camper because Scriv produced a cleaner .doc than Word—other Smashwords users actually had to clean up their Word-produced .docs, the poor devils.
Complicating this situation is the fact that Scrivener has the new .docx converters, and the Preferences->Conversion pane text isn’t updated to reflect what’s really happening with all the various options.
I’ve never used 2011-compatibility. I assume (incorrectly?) that it means little when trying to produce a legacy file format from 2003. I had the Aspose converters off (“Enhanced converters off”) and was therefore using the Scrivener docx converters. My first attempt used the “export as RTF-based .doc file” from the dropdown menu next to “Microsoft .doc:” which produced a nicely formatted file which Smashwords absolutely refused to accept. The error message told me that the file was corrupted, and suggested several MS Word troubleshooting techniques. Gee, thanks, Smashwords. 
My next attempt selected “Use standard MacOS .doc converters” which resulted in badly formatted files with all internal links stripped. No big surprise there, but I had hope.
I then tried .odt as an output format, but with the Aspose converters off, that reverted to standard MacOS as well, which led to the same result as MacOS .doc. Bummer.
At this point I gave up, used the Scrivener .docx converters, opened the result in LibreOffice, and saved it from there to .doc. Beautiful output file which I submitted and Smashwords accepted. Checked their conversion results: Yay! Correctly formatted ePub, .mobi, PDF, and online preview just in time to make Smashwords’s big winter promotion which started today, Dec. 25. (yes, I already have some sales! Yay!) At that point, I posted my original post.
I can use the Aspose converters (about to try them) but with Keith questioning how long MacOS will support him using them, it’s a bit nerve-wracking. At least my solution with LibreOffice will not be sabotaged by Apple. And to be sure, Scrivener has never claimed to be a do-everything formatting solution. Further, it is more than a bit frustrating that Smashwords have not moved with the times. (I will definitely drop a suggestion in their box. 
)
So at this point my question has morphed to—Dare I trust that the Aspose converters will remain? If the Aspose converters vanish, will Scrivener have any support for the original binary .doc format?
Scrivener gives three options for .doc binary files, [1] Enhanced Aspose exporter, [2] legacy RTF, [3] Legacy macOS (there is a Word-2011 compatibility toggle too). I assume you tried all three and none handle the feature set you require. I’m surprised that the Aspose option fails, as their converters are supposed to be good. I doubt Keith can do much about [2] and [3], but if there is a bug in [1] perhaps that can be dealt with? So when you use [1] what is the exact issue (doc cannot be read at all, or content is mangled)?
