How does the empire rely on strong individuals, when it seems like the individual (in its society) is subsumed in the multitude? How does it sustain its oppression and "evil", when such a system would presumably cause massive rebellion?
You're looking at it the wrong way mate. Sure enough, if you tried to create the an Empire's culture in any real life country you can name there would indeed be a massive backlash, our society is progressive, nuturing and generally good, and theirs is evil?
I say not, it appears that way from the outside but it seems to me that their culture is more like a hyper-charged version of the animal kingdom. Animals don't begrudge the more successful members of their species and I'd imagine it would be similar in the Empires, it's not intentionally evil simply the way it is. The kingdoms encourage liberty, and wealth, but they also allow and in some cases encourage laziness and mediocrity. This is why there is no decadence in the Empires, with it comes complacency. The strength of the Empires is most clearly displayed in the fact that the mediocrity of the Kingdoms' population is destroyed through natural attrition and (for lack of a better concept) darwinism.
This sounds about right. I would add, think, the Mongel Hordes. As individual groups they simply fought amonsgt themselves for local territorial control, but when a True Leader emerged among them, one with a true vision of what the Mongels could truly become, Ghangis Khan, the combine "Empire" of the Mongels became a force that not even the Romans "Kingdom", the Global power of that time, albeit somewhat in decline, could handle.
ehm, I wasn't aware that the Mongols fought the Romans... The Huns and Goths yes, but not Mongols.
I still think this social conflict between the Empire and Kingdoms sounds a bit naf. Maybe I'm a bit jaded by reading too much fantasy drivel but personaly I'd appreciate something much more if it was plausible, and this frankly isn't yet.
Perhaps it's just that I appreciate some degree of historical equivalence, i.e. that technology and society in a fictional setting broadly match those in history, e.g: JRR Martin's Song of Fire & Ice is broadly believable as an equivalent of the renaissance world. The current imaginings of the empire do sound rather like they have been coloured by Nietzche - modern, bourgeois and industrial philosophy.
My issue, if you will, is not that this is all wrong ro offensive, its just that it doesn't make sense. I find the narrative too close to history in some senses, and then wildly divergent in others. The inconsistency is the issue. Tolkien, for example, got around this by making the Orcs completely different, with their own inidividual and radically different kind of ethnicity and society.
I'd suggest making the Empire more different to make them more believable (less human, less similar to historical examples and hence less prone to historical/modern comparison).