I still could not update my website, so I did a fresh "Publish to folder" and used FTP client Transmit (
http://www.panic.com/transmit) to copy the result, i.e., one folder and an index.html file, into the Sites folder (located inside the Web folder) on my iDisk. Why use Transmit? Because for some reason, the Finder is excessively slow to copy files to and from the iDisk. Transmit is a bit speedier, though still takes a long time.
When I was done, I opened iWeb, and after a pause of about 30 seconds, it opened with all the icons on the left in blue, i.e., up to date. I then launched Safari and accessed my site, and lo and behold, it was there. I still cannot change anything; as before, iWeb goes through all the motions, but no modifications end up on the iDisk. Unlike before, I am not getting an error message; instead, everything looks perfectly normal, as though the changes had made it to the iDisk.
The procedure of using Transmit to make changes is a very poor workaround at best. I can use the Synchronize feature of Transmit to change only those files that have been modified, but the problem is that iWeb regenerates so much every single time one publishes (to folder or otherwise) that there are huge numbers of files that need to be transferred. In other words: it's good that the site is back up, but it's best not to update it until someone finds a solution to this problem or Apple publishes an iWeb update that fixes things.
Now I have to see if I can do something similar to restore my Galleries which have suffered the same fate. That, however, is a topic for the iPhoto forum.
Daniel