Edit: eshin'ed by boney, who makes a much more concise point than my rambling below, even if they are in large part agreement.
I was going to answer all of these in full but its half 1 in the morning here now and my mistake in my last post shows I'm probably too tired to answer these fully. Instead I'll go rapid fire bullet points if that's okay.
Rapidfire answers are:
- Its mentioned in multiple places that the druchii have both freeborn framing communities aroudn their city states and large nobility owned plantatio the southern side of they territory.
- They do have a hope for advancement via Malekith, the temples, the citizen armies, leaving the cities and becoming shades and via joining the Black Arc fleets or other raiding fleets
- Ditto here for leaving to become Shades or Reavers. Groups not under the Naggarothi state's full control but associated with it and used as military auxilliaries.
- Both slaves making items of "sub elven" make and freeborn artisans in the cities. Like most pre-modern states they are not industrialised.
- Aside from raiding the Druchii also participate in the slave trade, of which the only polities that have made slavery iilegal we hear of in canon are Bretonnia, the Empire and the Karaz Ankor. Notiably all polities with economic systems which already discourage chattle slavery. Ulthuan, Kislev and Athel Loren don't have chattle slavery but do have domestic slaves which show up in novels and are mentioned off hand in the wood elf army books. The Estalian Kingdoms, Tilean City States, Arabyan City States and Desert Tribes, Zharnagrud Empire, Norsca, Hung (chaos tribes north of Naggarond), Ind and Cathay all do have chattle Slavery (though with the exception of the Dawi Zharr are less reliant on it then the Druchii) and participate in the slave trade like the Cruchii. What little we know about the southland humans south of Araby also includes them participating in the slave trade.
- No the Druchii aren't particularly any nicer but as I hopefully illustrated they still get plenty of slaves. In addition to slaves from overseas raiding and trade they also enslave Hung humans and Naggarothi Beastmen, Greenskins and Skaven and also conduct trade with all of those non-state peoples.
- Originally the Druchii. It wasn't until centuries after they first landed and settled there they started shifting to a slave economy in full. THis ties into this question "Second, the scaling required to go from a decade or a century up to several millennia without major changes to a society, it's leadership, it's aesthetics, it's alliances, and it's enemies." they did have major changes. Its been 5000 years since the state of Naggaroth was founded and its has go through exapansion and contraction during that time and changed its economy, social structure and even government for the short time after the Battle of Finuval Plains where Malekith was absent.
- The fact that they have an immortal autocrat is that has been socially engineering them for 5000 years is also a big factor in their plausibilty to me. Really the its been 5000 years thing is very important to my SoD here. Yeah they more then tripled their population. In 5000 years as a settler colony society with land "up for grabs" and enslaved natives for labour.
Edit:
Depends how you count cities
By definition the polis of Sparta was a city (polis) in the context of time even though it never got beyond what we would call a large town. But again that large town at one point (about 50 years in length) was the hegemon of the Aegean Sea. I don't like overhyping or lionising the spartiaes or Sparta anymore then the next anarchist-communist but they where not exactly without their political and military victories.
The impression you are giving is of a population that largely consists of freeborn farmers, merchants, and artisans with one culture- that needed for the creation of communities and the raising of families- and another, smaller population of extremely decadent and bloodthirsty nobles who live forever and maintain their power through naked force?
That's plausible, and I think that if we accept the dark elf descriptions in the books to apply to like the aristocratic 5% with everyone else running on different patterns, it would solve most of the issues I have. It also fits in with the 'not industrialized' bit, requiring most of the population to be doing primary production. (Food, raw materials)
The trick is, then, we need to agree that the books only describe the top fraction of druuchi society.
I do disagree with you fundementally about possible advancement through armies, churches, Malekith's personal service, Black Arc fleets... All of it, really.
We know that elves live a REALLY long time unless they are killed. We also know that most of the people who die in wars in this sort of era are the front-line grunts: get promoted a few times and your chance of death in battle nosedives. Combine these two things and you've got the hick county elves joining for a few years, realizing ambition has no path that isn't already occupied by paranoid elves scheming to create empty slots above them, and take what they've made back to the county.
And this actually makes the sheer poisonesness of druuchi politics make sense- IF you are playing the game then the only way to open up slots above you is murder or exile. Like playing musical chairs. Malekith just needs to enforce a chain of command with himself at the top, not caring who fills what slot underneath as long as it is filled, and the rest kinda takes care of itself...
Which also solves the issue of "How does an immortal warrior-magus actually know anything about social engineering?" I dislike solutions that require villains to be vastly competent in all areas. Sculpting a society to your whims is not something that I think is at all simple or straightforward, or even amenable to experimentation. It's heavily path-dependent and driven by second or third-level derivative effects of your actions, not by your intent.
I'd note that anything not under the state's full control (shades, reavers, etc) is probably going to be harshly and regularly purged. It's possible that the purge night grew out of Malekith winnowing through the druuchi he didn't fully control in search of the ones he didn't think he could trust? Seems like the sort of thing a paranoid King would do, plus a few thousand years of all his actions getting sanctified?
But ultimately, again, this is me assuming that the farms and estates make up most of the population and get basically no attention, the crafters only a tiny bit more, and the few soldiers, priests, and aristocrats are almost alien to those underneath them despite defining the entire civilisation to everyone else.
Which, writing it out, sounds ENORMOUSLY plausible.
But it also means that the vast majority of dark elves are probably decent, hardworking people. And the only really odd thing about the dark elves then is the way that they'd never have had a chance of a revolution succeeding, so we'd see a society that evolved to basically form a cyst around it's toxic rulers.
So the open question is what the slave/freeborn elf population ratio looks like. I'm still, despite your arguments above, very skeptical that there could actually be that many slaves in Naggaroth, mostly based on how fragile people are and how mind boggling horrific the death tolls were for the middle passage in reality- most people, the vast majority of people, die to the kind of treatment that happened in reality, much less the caricatured extreme version of it the druuchi do. I figure for every hundred thousand captives taken, maybe a few thousand make it to Naggaroth? Enough to fill pits and mansions in a few cities, but past that... And farm labour under harsh conditions kills quickly too.
As a side note, given that the only thing the druuchi seem to really have to buy slaves with is... more slaves? I'm not sure how they would make up for the low numbers available from raiding with buying them from other polities, even if most of the polities in the world practice slavery. (And somehow set aside their hatred of dark elves to do business.) What do dark elves export?
...I'm going to ignore the claims about dark elves changing over the years unless you've got examples. I don't think eight years of Malekith being gone out of five thousand counts as any sort of dynamism.