That only works if you have imported the image files. For most users, the covers are there but are not actually imported into the library.
Thanks for pointing that out. In my workflow, all album artwork is saved both in the flac file headers and as sidecar files in album directories. Then I always import both audio and image files into my library. As you say, that is not necessarily the case for other users.
Also, "Total Tracks" is a tag like any other - image files usually won't have that tag set to any value. So the smartlist just returns audio or image files that don't have "Total Tracks" set, or it's set to zero, regardless of there being other audio files in the folder.
Per the library field manager, [Total Tracks] is a built-in non-relational audio-only field. It is usually auto-filled when MC rips a CD; otherwise I set it manually. However, as you imply, [Total Tracks] is
not updated automatically if the track count changes on disk.
FYI aside - new work flow: Because [Total Tracks] is hard-wired with Flags=Audio, it is not visible in image views. To make the info globally available, I define a custom album-relational, standard edit type field [Total Tracks UNV] (UNV=my code for "universal"), with all Flags set. I manually initialize it to same value as [Total Tracks] in my audio tags panel. MC then auto-writes it to both audio and image files (MC writes MJMD tags when the image file format allows it). Note: I do not define [Total Tracks UNV] to calculate [Total Tracks] in the library field manager, since MC reverts the former to non-relational in that case.
One reason why my last smartlist revealed directories without audio files is because those directories were set up that way intentionally (for example, an "extras" subfolder which only contains images and documents). The smartlist also revealed data entry errors where "Total Tracks" was not set, as zybex mentioned.
Conclusion: As far as I can tell, zybex is correct in that, sadly, the answer to OP 2) appears to be "no".
[edited 6/2/2022]