As I read it, "fifteen degrees behind the planet in the same orbital path" would mean that it's in the same orbit as the planet but trailing it by fifteen degrees -- i.e. in the same place that the planet itself was 1/24 of its year ago. If Titan
was coming into the system from the same direction, there's no reason that point couldn't be between it and the planet.
So it's not "behind it" from Titan
's perspective, but in the sense of its orbital motion.
As for Keru, as far as I know, he's the security chief and Tuvok's the tactical officer.