Hi. I removed multisite by following the instructions layed out in several blog posts such as http://wpmututorials.com/how-to/how-to-disable-multisite/
I can access plugins, but there is no Delete option for any plugin, and there is no Add Plugin menu item.
So it seems my site is a single WP install, no longer multisite, but it is confused and won't allow me to properly manage plugins.
If I don't come up with a viable solution I guess I can always re-enable multisite just so I can manage plugins.
BTW, I migrated a multisite primary blog over to a single WP install using BackupBuddy.