As far as I can understand, when the first Sindria country was attacked (with Judar mixed into in, details not revealed until now) many people died, including some major characters of Sinbad adventures... and at the time, a lot of black rukh was released and "infected" the dead and the dying people.
Somewhat at the time Sinbad knew that remaining like that would be hell for the people he swore to lead and protect, so he took the black rukh to the only place it wouldn't harm them - inside himself. His white "core" (the round thing we saw when he was in Focalor's badass mode) was colored partialy black and then he had to re-learn to use his rukh (with the Magoi manipulating tribe).
After that he started to hear a voice - David's voice, but only after Alladin explained Solomon's story was when Sindas realised what, and who, David was.
I don't think he is David's reincarnation as in the sense he was David in the past life - in fact, David and Arba (after the initial part) treat Sinbad and David like two different people (even if in the end they end up as simply 2 diferent pats of the same soul). So David only appeared after Sinbad was corrupted - so there is a chance that in the beginning Sinbad was just himself (and I think there was a interview with the author that supports this), it was said that the read David's reincarnation died that day in Sindria and when Sinbad took the rukh, he was taken along the ride.
What exactly singularites are in not well explained and David himself said that were others before him, but who didn't make an inpression on the world like he did, and he didn't treat them as if he was them in thhe past.
As fas as controling Sinbad, I'm sure David would love to, but he has no real and effective way to do so untill now. He is just a spirit that shares Sinbad's body with him, but Sinbad is the owner of the body, and is strong enough to hold him off for now (David's silence promises something) and David can only talk with him, convince him wih words and not force him to do something he doesn't want. And I don't know if its because the darkness is not his to begin with - but an external force that was mixed into him - yet that may be why Sindad has remain mostly rational and loyal to his own morals and dreams. But after so many years with the darness corroding him, he may have started to change halfway.
The only way to know more is to read future chapters and I really hope we get to know more about what and how Sinbad (and David by extention) was born.