What I would do is to set the number of the hymn into an anchored frame, so it can be defined exactly in relation to the spine. Also other objects or frames which have a fixed relative position to spine or not to spine.
I am afraid I don't understand what that means. :-(
Why do you use 2 independent frames? Make it into the same frame.
Because the hymn numbers always need to be on the outside of the page.
Another method would be to let the songs completely run in a story.
What is a "story" ? I would love to know ... sounds very powerful !!!!!
Question: How did you set the notes? I have often to typeset christian books and there is often a hymnal part in it.
I use Sibelius. I have also used Finale, but I prefer Sibelius.