Your questions are really interesting, and hopefully, someone might add their knowledge to what we say here!
Regarding the music that is used in the Monogram films, I believe that the only one that used unique, original music was "Charlie Chan in the Secret Service. The Musical Director was Karl Hajos. Everyone will agree, I am sure, that the music heard was truly unusual in its "booming" nature!
When I spoke some years ago with Noel Neill (or it may have been Elena Verdugo!), who was in "The Sky Dragon," she told me that the actors who appeared in several films made at about the same time would just come in to work the scene or scenes that they were to appear in, and that was that. They didn't hang around while other scenes were shot. That is how we can see them in all the other movies from the same time period.
