It seems you are not interested in using a virtual tag for this. And looking at my suggestion, I see that 1A vs. 11A would indeed pose a problem there too.
As an exercise, and perhaps for others stumbling onto this thread, I adjusted and completed the formula. This should work:
$IsNull(<Initial Key>,,$Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace(<Initial Key>,11,C),12,D),1A,"A♭ Minor"),2A,"E♭ Minor"),3A,"B♭ Minor"),4A,"F Minor"),5A,"C Minor"),6A,"G Minor"),7A,"D Minor"),8A,"A Minor"),9A,"E Minor"),10A,"B Minor"),CA,"F♯ Minor"),DA,"D♭ Minor"),1B,"B Major"),2B,"F♯ Major"),3B,"D♭ Major"),4B,"A♭ Major"),5B,"E♭ Major"),6B,"B♭ Major"),7B,"F Major"),8B,"C Major"),9B,"G Major"),10B,"D Major"),CB,"A Major"),DB,"E Major"))