I think it would be a good idea to move the set cover art functionality to the edit release group page. Most people are completely unaware that the existing link even exists and there's no real reason it needs a separate page.

I've attached a mockup of how I would probably implement it. When you click the "change" button, it would either open a bubble like when editing artist credits or a dialog like in the relationship editor, which would contain a list of releases to choose from, like on the current set cover art page.