Now my ability to see flash files—well, the ones I want, like YouTube vids; the ads seem to work just fine, oddly enough—is borked.

Presto, I tried so many different things that I couldn’t begin to retrace my steps, let alone distinguish progress from superstitious behavior. What I did do just before getting it to work was this: I uninstalled Envy (which had many dependencies that weren’t automatically flagged), then entered “System Settings”, got admin privileges, and tried (for the hundredth time, it seemed like) to enable the nVidia driver. And that time it worked. It hadn’t worked before I attempted to install Envy, so why going through that sequence made any difference is beyond my comprehension. I don’t think I downloaded anything else that might have helped, but that’s possible.