Thank you for the question and for sharing the print you are working on. (And I'm so glad you thought the workshop was wonderful!)
The Guidance for Transcription Using the Nemeth Code within UEB Contexts (Approved April 2018) does not address authors' comments to spatial material. What we do know is that Nemeth Code must be in effect for the whole spatially arranged long division problem. So, I cannot think of a good way to transcribe the author's comments, which must be mostly in UEB, as they appear in print (i.e., next to portions of the division arrangement).
If these sort of speech bubble comments appear throughout the book, then it might be practical to precede each such comment with a transcriber-defined shape indicator (e.g., ⠈⠫⠎⠃⠀) and then present them consistently before or after the material to which they apply in some consistent format (e.g., 7-5).
Alternatively, using #16 under Formatting in the Guidance as inspiration, another approach might be to transcribe the comments "on the line following the required blank line in the displayed position for that text". An example of a system of equations transcribed according to this portion of the Guidance is Example 9 in this year's "Nemeth and Formats and How They Work Together" material, which is available for download in PDF and BRF formats from NBA's Conference page.
In any format, I think it will regularly be advisable to use embedded transcriber's notes to specify what the comment refers to. For example:
A repeating decimal has decimal expansion that repeats the same digit, or block of digits, without end. ⠈⠨⠣⠉⠕⠍⠰⠞⠬⠀⠕⠝⠀⠸⠩⠀⠼⠴⠨⠆⠶⠶⠀⠸⠱⠈⠨⠜