Twenty different people (thank you, twenty different people) have sent me one or both of two links that fill in a few pieces of the Sandpipers riddle I posted last night.
At this site, we learn about the Sandpipers group that recorded "Guantanamera" and other non-kiddie records. They do not seem to be the same Sandpipers heard on the Little Golden Records.
This link goes to a page on a Gilbert and Sullivan discography referencing a Golden Record. It would seem to establish that the Mitchell Miller in question was the same bearded gent who fronted Sing Along With Mitch. The Sandpipers, it says, was the name of a chorus that he used on his records and TV show. The reference to "reliable studio union New York Singers" suggests to me what I'd suspected, which was that there was no permanent roster; that Miller (or whoever) hired three or four guys each time, and that while they presumably tried to hire the same crew for each session, if one wasn't available they probably plugged in another vocalist.
Okay, that answers some questions. I'm curious as to whether Miller owned the name of the group and I'm really curious about the version of their song that turns up on the Man on the Moon soundtrack, but maybe some of that will get answered in the next day or two. Several of the twenty said they think they can locate additional info. Check back later.