Couple of points:
(1) You are right that first year starters generally tend to do pretty well in college football, and I don't necessarily hold that as a whole against UGA. You have a decent (even if not great) QB with a strong supporting cast around him -- which basically is the entire list that you have, sans Newton and to a lesser extent Winston -- and that's generally enough. All of that said, I'm not sure any of that really applies to Georgia this year, because the OL is being rebuilt and because there is no true star at WR, not to mention the fact that the defense is still unproven (even if talented with a promising new DC on the way).
(2) My point on 1998 Tennessee wasn't necessarily pointed directly at the Manning-to-Martin succession, but instead more generally just to the entire course of events that somehow all fell in their favor that year. They started out with a much weaker SEC in those days, then they had squeaker wins over both Syracuse (which was a middling team) and Florida, and then the literal Hand of God play to bail them out against Arkansas. Of course, then the Vols get a weak MSU in Atlanta, which looked poised to pull the upset before a fourth quarter meltdown, and then they get FSU in the BCSNCG facing what amounted to a third-string quarterback. Regardless of what you have under center, that's just a really strange, wacky set of circumstances that is incredibly atypical, and one that really no one could ever expect to fall in their favor.