Thanks, folks.
I decided to take a chance on a 12.2 image that's about the same rev as the one I destroyed, and which seemed to be a real copy. So far, it appears to be legit. I'm probably missing a few fixes, but this is just a "class project", so it's no biggie.
I also discovered, from reading the experiences of others that made the same mistake, that "startup-config" isn't actually a file name: it's a keyword parameter, subject to the same "minimum match" rules as other parameters. You can do a save without the risk of shooting yourself in the, um, flash by just typing:
copy run start
The command interpreter will then feed you the right default filename, and do what sensible people >:( would expect.