I am not confident I can formulate an answer to this question.
First, please let me make sure that I understand the situation. You're transcribing mathematical material that is written in Spanish? You're using uncontracted UEB (including UEB modifers (aka accents), UEB punctuation, and UEB typeform indicators) for the narrative text? And you're using Nemeth Code (surrounded by code switch indicators) for the mathematical material? [And the reader is familiar with this code combination?]
Second, please let me make sure I know what you are asking. Are you asking about letters that are standing alone in the narrative text (i.e., in UEB)? And, are you not asking about single-letters within Nemeth Code (i.e., inside Nemeth Code switch indicators)?
Thank you for clarifying!