I tried both suggestions, with no joy. But --
When I returned to my normal user account and opened the file again, I found the chapter headings in the body of the doc (to which the heading 1 stile was applied) had disappeared. The document map still showed the "ghost" entries that linked to the right
places in the document, but now the actual text of the heading was gone from both the document map and the body of the doc.
I'm not drinking, I swear.
I deleted the "empty" line with the heading 1 style from the document and re-entered it. This time the text appeared in both the doc and the document pane, except the incremental chapter number was one higher than it should have been.
After poking around a bit, I found that I had to delete and re-enter the preceding page break that separates one chapter from another -- then all was well: right chapter number, text visible in the doc and document page, as expected.
Sorry for all the confusion. I don't think I'll ever know why/how things went south, but at least I have a somewhat tedious process that seems to clear everything up.
Now I only wish I had a way to check the overall health of the document. I have this lingering suspicion that the root cause was a damaged document and I'm not sure adding the headings back will take care of that, if that's the case.
Regardless, many thanks to all who responded so quickly trying to help.