Good afternoon, Anna!
Thanks for your question! There really isn't a publication out there about formatting music books such as this. The Handbook for Music Transcribers is not updated to either UEB or Music Braille Code 2015 as of yet. It is helpful, but there are things that have changed. The best thing to do is study Formats and look up other music books (if you can) that have similar layouts and see what others have done.
As far as the example you've attached here, things look good for the most part! Code switch indicators are not really needed on the A) B) and C) letters before the rhythms. I think the list format and the context make it clear enough that the code switches. But the margins are fine and make it clear what's going on.
In the music heading, where you have three different tempos, I would treat the "Basic" "Advanced" etc as tempo markings and put a period after each and then the dotted quarter = 80. Start each one on a new line and you can omit the semicolon. If you feel its crucial to have them in there, you don't need the punctuation indicator before the semicolon, as you're not entirely in music braille code at that time. (The numbers are numbers, not notes.)
(Don't forget your key signature with your time signature. Also, each measure does not require an octave indicator in bar-over-bar format. See MBC2015 section 33.4. Also, has the customer requested uncontracted braille with this? In most method books I've done, I use contracted braille except for the text of the music heading and anything within the music.)
I hope this helps! Let me know if you have more questions!
All the best,