Can't comment on TMM I'm afraid, having never used it (really must try them all out !!)
I set my stuff up with MediaElch, although I also had an issue getting the correct movieset art to show.
As an example, I have the "Batman Original Anthology". so, in my movie folder, I have one called "Batman Original Anthology". Inside this are four folders (one for each film) and the poster art, clearart, and fanart for the set. Each individual folder for each film then contains the actual .mkv and the associated art for it.
The art for this top level is called fanart.jpg (for the backdrop), folder.jpg for the disc cover, and logo.png for the clearart.
I am aware of Movie Set Artwork Automator, but sadly in my case, it didn't do what I wanted it to.
Artwork downloader (available in the programs section of the official repo) is the only method I have found to reliably add the art correctly.
This is how it looks in my library.