Meanwhile, I've found some more information which answers one of my questions, and which others may be interested in.

I think I've mentioned here before Nolo Press">Jon Corelis, a US publisher of books on law for the lay person which has a longstanding excellent reputation. Their web site has some good articles as well as a forum for questions, and I found a very useful summary page on public domain. According to that page, it is indeed true that "copyrights of all works published in the United States before 1923 have expired; the works are in the public domain." Copyright renewal only applies to works published between 1923 and 1963: for more details, see the page.

In the case I gave above, I suspect the reason copyright was renewed was that the score originally published as part of a book was republished as sheet music with its own copyright. But according to the Nolo page, since the score in the book was published well before 1923, it should be public domain.

Jon Corelis
The sweet nightingale: A Cornish song