Apparently someone had a lot of fun making the print in this book as visually appealing as possible. *flat stare*
This 5th grade book has different shapes around problem/exercise numbers, around multiple choice answers, and around the numbers related to the step-by-step explanation. To my knowledge, the closest BF comes to addressing this is where it says to use transcriber typeforms instead of the enclosure symbol. But that seems a really weird use of typeforms. Showing just the number/letter is also weird, because that makes it slightly more difficult to identify that number/letter as a problem identifier.
I almost want to include a note on the TN page saying that "Step It Out numbers are enclosed in a green pentagon-shaped arrow in print, and followed by a closing braille grouping indicator in braille. Problem numbers are enclosed in a green square in print and followed by a period in braille. Multiple choice answer letters are circled in print and followed by a period in braille." This way I can indicate the difference between Step It Out and problems, and still differentiate between the problem identifier and the problem without adding excessive braille symbols. On the other hand, I don't know of any portion of Braille Formats that would call this permissible (except the catch-all "transcriber discretion").
I agree with the way you handled the question and answer problems - putting in a TN that they are just simple numbers and letters followed by a period in braille makes perfect sense. As for the Step It Out arrows - are the arrows necessary? I assume every Step It Out is preceded by the heading - so it is pretty clear that's what it is. You could just put in the number and NOT follow it by a period - and put that in the TN. Or enclose those numbers in parentheses. This would use more "familiar" symbols. The way you chose certainly isn't wrong as BF does not directly address this issue that frequently occurs in print. Choose a way and then be consistent. The reader will understand. What a challenge you have!