The "Message" link is now dead. Maybe he took the pdf down to change stuff.
I do have the Hal Leonard "Message In A Box" music, but it's for guitar and bass. Add drums and you have the equivalent of the Beatles book, which I also have. My only thing for both of those sets is that the lyrics are really jacked. Whoever did the MIAB set did not have proper access to lyrics because I can't believe the mistakes in those (some are on the Synchronicity tunes where the lyrics were printed from album to CD!).
For the Beatles book, it was Japanese to English so that may account for some mistakes (there are others like "hold you in his armchair.." which people still think is the lyric in Come Together). However, for the incredible work they did on the Beatles book there's a lot of musical mistakes. First thing that comes to mind is the piano part to "Martha My Dear" which is transcribed incorrectly.
Don't think something like "Darkness" would need two staves. Just clarify the delay and the note value (e.g., 8th note delay in the right channel). You WOULD need extra staves for "Message In A Bottle", especially for the last verse--obvious overdub.