I think there's enough information to believe that Nicholas (Nikolai) was probably his formal first name. But one can only say 'probably' because there are at the same time enough references to Johnson being Brian. As GD Michael's secretary he was a known quantity, so to speak. Further, one simply doesn't just come up with a new or false name out of the blue like this, especially as there would have been virtually no other Johnsons (or Brians) in Michael's circle with whom to be confused.
And with the patronymic "Nikolaevich" we know his father was a Nicholas, not a Brian. And so the mystery remains. Quite frustrating.