Some versions of the film use the original names, however these were all put in much later than the films initial release. So while the character may have had his name changed back to Dracula in some editions, it was Orlok in the earliest incarnation, thus it is Orlok that 99.99999% of the people who see it refer to him as. Mark Ellis wrote a graphic novel called Nosferatu: Plague of Terror that expanded on Orlok's story, making him a seperate, and perhaps more powerful entity than Dracula.
As for Olrox, I think he was some kind of failsafe. If Dracula was running low on energy, he could kill Olrox and steal his power. It's probably the same reason Walter kept Joachim. Of course because both of them were killed prior to the final battle, their purpose could not be utilized.