For the questions on fair use of the scans: do the scans add something to the article that you really just can't get any other way? With album and book covers, it's *the* cover for the album. Is a score regarded that way?
It's functionally equivalent to a short sound clip, without the additional burden of being one person's interpretation of the work. Or you might liken it to a photograph of an individual. It is a picture of the work described.
Does it add to the article in a unique and meaningful way? I'd argue very strongly yes. If you read music, the excerpt tells you an enormous amount about the work. The out-of-copyright ones, which would mainly apply to Victorian music hall performers, are very unlikely to be rejected from those articles.
I would say it's equivalent to including an extract of a book in the article about that book. Would we allow that (beyond quoting a specific passage when discussing that passage)? I doubt it...