Bere with me, I'm still trying to get my head around some of the code base. The goal at the moment is to get a working version into extras, no extra for the time being.

Let me make sure I get what you mean by "in "album art" mode, while editing single album"
Open Mussorgsky, click Album art, select album, wait for results. Then clicking a cover doesn't set it, the dialog just closes. Correct?

With respect to album cover results being incorrect, as I understand it at the moment it looks at metadata to see if the album and artist or just album are set and uses them to create search term originally used to display results. It then just takes the first few results given by the search engine. I've often had to modify the search term to get the correct covers, be it for deluxe editions or country specifics to name a few.