I think Duran was a Xel'Naga as well. He said that he was a servant of a "far greater power that has slept for countless ages," and the only group such as that would be the Xel'Naga. Because of this, I don't believe that Duran had become infested by the Zerg, but instead he simply changed his appearance and person to play both sides as he did with the UED and Kerrigan. His voice also changes in the bonus mission, possibly to its true form.
I'm actually interested in what mind happen to, say, Stukov, if he might return infested like Kerrigan and pose a threat to her power in the K Sector with a splinter group of infested Terrans and other Zerg. Needless to say, the BW ending was a terrific cliffhanger, and I'm still anxious to know what would happen to Raynor in particular.
Mods, could we possibly split the SC discussion if this gets any larger?