Your lines
<< \bckngNotes \gtrBackingNoteDetails >>
should be
{ \gtrBackingNoteDetails \bckngNotes }The double angle brackets are indicating that the music should be simultaneous like voices, which isn't what you want. (b1 and s1 happening simultaneously in different voices) And you'd want to use \set before your music. Also then you could remove the s1.
