Hi Lucas!
GSTG 2022 adds some information to 5.8.1.3.
“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.”
No grade 1 indicator before a two-letter key, even if the key corresponds to a shortform word.
This points to your choice #1 is the right option. Use the single-letter key followed by a period and do not include the grade 1 indicator.
Hope this helps!