We called PayPal again, this time we got somebody who really seems to know what he’s doing. The fix was applied right then instead of the previous “it should work in a day or two.” Immediately after, my wife (who previously tried and failed to donate) tried, and it worked.
So it looks like the donation thing is really fixed this time.
Of course anyone who wishes to, is more than welcome to. If you get an error message, do let me know (and copy/paste the error message in your comment).
Thanks very much to all who donate, and to all who’ve helped track down the problem, and all who have been patient.