Thanks for all the suggestions!
Unfortunately, none of these worked. All the attempts yielded the same result, that being that anything I attempted to deactivate claimed to be "deactivated", but in reality never actually was. Resetting the plugins folder did manually deactivated everything, but when I set the folder back the way it should be, the original problem returned.
Related, I'm similarly unable to switch themes (so unable to try your 2nd suggestion): when I click "Activate" under a specific theme, the page reloads and claims the new theme has been activated, but in reality the same theme is still activated.