As a first pass review:
Needs a midi section (I did a quick one with 'choir aahs' for vocals and 'acoustic grand' for the piano, \tempo 4=100, and it sounded quite nice.)
You are going to great lengths to keep this to 2 pages. This is fine but Mutopia will want to create both A4 and Letter sizes so your paper variables (2 A4 pages) will be commented out to accommodate this. On letter size your manual breaks also will not do what you need them to do.
A suggestion: design for A4 but leave out the set-paper-size command and remove manual line breaks. Remove set-global-staff-size so it defaults to a little larger ( it will go to 2 systems on the 3rd page instead of one.) Request (or add yourself) a Mutopia 'moreinfo' tag in the header with text that the layout was designed for A4 paper size.
-glen