I'm not sure if the focus of this discussion is clear to everyone. As I understand it, the primary concern is with preventing vandalism of prominently placed images, such as those on the Main Page. This is handled by protecting the images. I take it that this is the only administrative function UninvitedCompany (and others who would like Commons to adopt more liberal adminship policies) proposes to use there. Nobody is going to be going on blocking or deleting sprees.
This would be a good candidate for a technical solution, specifically using the MediaWiki feature of user access levels. If the only adminship function you need on Commons is protection, for the purpose of protecting another project from vandalism, then it should be routine to grant (only) that function to anyone who's an admin on said project. It might even be possible to make this automatically part of adminship once single login is implemented, but at that point I feel like I'm wandering into pipe dream territory.
--Michael Snow