^Umm, the point is that "Khan
" was originally a title for Mongolian rulers going back nearly 2000 years. When Temujin took the title Genghis Khan and built the largest land-based empire in world history, he and his successors (who also used the title Khan) thus spread that title to other cultures and it came to be used in them as well, which is how Khan came to be a common surname in the Islamic world.
The mistake being made above was the assumption that just because Khan originated as a title, it must have been a title in this case -- that the character was named Noonien Singh and simply used the title Khan. Which, as I pointed out, doesn't track, because then he would've introduced himself by saying "I am the Khan" rather than "I am Khan." He and others have always used it as his name, not his title.
As for Caesar
, it was originally the family name of Gaius Julius Caesar -- more specifically, the cognomen of the family Caesar within the clan Julius. As for how it became an imperial title, see here