So I just finished up doing everything you suggested, jnhghy – a mixed bag resulted.
After updating all plugins and themes (even deleting several installed broken themes that were missing a stylesheet), deactivating each plugin, and then reactivating them one by one and switching to a Twenty Ten theme (the earliest I could find), everything seems to be working EXCEPT for the text – there is no text; but the 404 error doesn’t seem to be occurring anymore – which I guess is a relief.
Any other suggestions?