Thanks for replying. Great questions. I'll try to explain...
What would be the criteria/logic/condition that needs to be met in order for a folder in A to be deemed as having missing tracks?
Yeah, this is the biggie right? In mediamonkey there is a third party script called Tagging Inconsistencies. One of its options is to find albums with missing tracks. I am not 100% sure how it does that. I guess it could look for missing track numbers, but that would miss tracks at the end of the album. Perhaps it looks up the musicbrainz or discogs id, and then checks against that.
And suppose we get a way to determine the above, how would we know that the missing tracks are located under what folder under C?
Well, the only way I can see it working is to search the whole folder for each track that is identified as missing. Presumably by matching against id3 tag data. Perhaps you could use musicbrainz or discogs metadata here as well to identify the missing tracks.
Seeing there's a mention of it, how does folder B come into play?
Folder B was intended to be an example of a complete "known good" folder. It probably confused more than anything else. Apologies for the extra noise.
Perhaps specific examples (album or title tags, etc.) may shed some more light on how this may be achievable (if at all possible).
The Album "Whatever People Say I am..." by Arctic Monkeys is missing track 7, Riot Van.
For the sake of argument, let's say that track that was on many various artist compilations albums.
Run the plugin/script > identify the missing track > scan a target folder for tracks that have the correct id3 for title & artist > rename album artist and track# accordingly so it now completes the album.
Essentially this is cleaning up the library by completing albums wherever possible.
I hope all that makes sense. Please let me know if any is unclear, and thanks again for the kind response.