I would like to jump in and point out some things. -> Khan as title
There is already an existing precedent that a name can become a title, both in real life and in Star Trek:
IRL: Octavian and Caesar - I understand that they were not first names, but some sort of nicknames (sorry my Latin is a bit rusty)
ST: Seeds of Dissent and Debtors' Planet - alright nothing canon, but in Seeds of Dissent we have the "Khanate of Earth" ruled by "Khan Imperator" and in Debtors' Planet the "Great Khanate".
Therefore, I do not believe that it is such a scratch that Khan Noonien Singh's first name became a general title, given that it symbolized a ruler.