Justin was definitely cozy with the Outsiders, but I wonder whether he was an infected pawn, a uninfected dupe or simply using them for his own schemes. It seems unlikely that a former veteran warden would be a mere dupe.
Moreover, the fact that he had "adopted" two Starborn Wizards -- in fact, the ONLY two Starborn Wizards -- suggests that he was attempting to use the Outsiders for his own schemes. The odds that this was by accident are astronomical (see what I did there?).
The counter-argument would be that if the Outsiders can twist Starborn to their purposes, then Justin could be a dupe or infected.
It seems pretty clear that any mortal practitioner with enough juice and knowledge can summon Outsiders, not just starborn. In fact, if only Starborn could summon outsiders I feel certain that the White Council's reaction would be summary executions of Starborn, not the minimal watchful waiting they attempt with Harry. (Clearly the WC has no real knowledge of Elaine Mallory).
We also know that neither Harry nor any starborn can be mentally tainted, possessed or manipulated through supernatural means. Of course, more mundane manipulation would be possible.
We also know that Starborn wizards can more readily damage Outsiders, although Senior Council level Wizards can do ok.
All of this seems to be consistent with Starborn being good insurance against Outsiders. They cannot be suborned by them -- which is not true of most other servants or lieutenants like Aurora, Maeve, Lea, etc. -- and they can fight them more effectively, all things being equal.
Not only Justin, but Mab, Aurora and the Fomor have employed Starborn as enforcers, servants, lieutenants.
Putting all this together, I lean more towards Justin potentially using the Outsiders, but being independent of them, and wanting to be able to fight against them if need be.
