We would like to amend our earlier response to your question. The following adds more specific details from GSTG that will apply as you and your team make a decision on how to proceed in this case.
To start, regarding the use of switch indicators, while it is preferable to not switch multiple times it is not wrong if it becomes necessary due to the content of the key itself.
In Unit 5 of Guidelines and Standards for Tactile Graphics (GSTG), the rules for use of a key are included under 5.8.1 Keying Techniques for Transcribing Literary Material, and 5.8.2 Keying Techniques when Transcribing Using the Nemeth Braille Code for Mathematics and Science Notation, 2022 (and current updates).
In the Word document attached to the original question that was posted, a single letter followed by a period is used for the key entries. This use can be supported by Guidelines and Standards for Tactile Graphics 5.8.1.3 as follows:
<p style="padding-left: 40px;">
Single-letter keys are not recommended, but if used, they must be followed by a period. Do not use the grade 1 symbol indicator before a two-cell alphabetic key, even when the letter combination corresponds to a braille shortform.</p>
In order to comply with both the Nemeth and the Tactile Graphics guidelines, the key itself should be prepared according to those rules. Only after the preparation of the key, do we address the decision on how to best handle the Nemeth switch indicators.
If the entire key is to be contained within one set of switch indicators, then the key should be prepared with either a 2 or 3-cell alphabetic key, or a numeric key.
If keys include a single-letter key followed by a period, the transcriber would have to use multiple switch indicators to surround the Nemeth description of the key symbols. This option requires more code switching for the reader.
The use of a single letter preceded by an English letter indicator would not comply with the rules of either code.
We hope this is sufficient to guide your team toward a good solution in this case!