Which is why I mentioned the other possibility, as a form of wildlife preserve with elves in a natural setting.
That would be a potential explanation for not enslaving the Owl Clan as a whole, but it does not explain leaving a wanted terrorist alone when they know where she is, and they also know there's literally nothing the Owl Clan could do to prevent her being taken. It's not like their "nature preserve" is going to be negatively affected by removing and imprisoning a single elf.
It's also a really poorly maintained nature preserve, given the fact that poachers apparently keep flying in via helicopter and abducting some of them. And also, you can apparently sneak military grade automatons into the "nature preserve" without anyone noticing.
As for the number of elves versus humans there.
That is a smaller percentage than the ratio of free versus slaves in the southern states during the U.S. Civil War. Approximately 9,000,000 free in the confederate states, versus Approximately 4,000,000 slaves in the confederate states.
1/3 slaves in Sylanar, versus almost 1/2 slaves in the confederacy.
Remember, it was not 1 in every 3 people owning slaves in Sylanar, there were huge factories and farms using slave labor, along with mining operations, all of which would have been able to use a huge number of slaves.
And they would have been owned by only 1, or maybe a few people who owned or operated those businesses.
I'm not sure why you think any of that's relevant to the point; regardless of "free people vs. slave" ratios, or who owns the slaves, and in what quantities, it remains true that increasing the slave population by a few hundred is an utterly negligible amount, and would have precisely zero effect on the market value of a typical slave.
Also, remember Ashley saying her family was so poor, they couldn't even afford a slave of their own, and so one was provided by the government? If "can't even afford a slave" is a metric that defines someone as “poor”, then that kind of implies that slave ownership is fairly common, wouldn't you say?
We have a population of female elves of 500 million (500,000,000). Despite the elves longevity, we're still going to go by the age metrics as the youngest and eldest elves aren't capable of child bearing. So we're looking at the Elven equivalent of the 15-24 and 25-64 age groups (or 57.79% of the population)
There's a pretty massive flaw in your premise, right there; you're making the assumption that their population would be roughly equivalent to humans, in terms of proportional age ranges, and that's absolutely not the case. Those "eldest elves" would have to be almost a thousand years old to be past child bearing age, if we're talking about common elves (who live fifteen times longer than humans, and so would need to be about 975 years old to count as equivalent to a human of 65). Given that humans arrived on the continent only a few centuries ago, and one of the things slavers tend to do is "kill the elderly, as they make poor slaves" there's probably vastly fewer elder elves than you think. And there have been three violent slave uprisings, so large quantites of the adult elves who managed to survive the original invasion were probably killed during those uprisings, and any common elf born since then is still the equivalent of a child/early teen.
Lin was a very young kid when humans arrived and is equivalent to a 20 year old human, so every full-blooded common elf born after that point would have to be even younger than Lin is. As for the half-elves, Mariella is one of the oldest half-elves (Runey said she was 315) but is the equivalent of a human in their early 40's. Dark elves may have half the lifespan of a common elf, but that's still seven and a half times longer than humans; the youngest dark elves born into slavery are the equivalent of early 40's.
All-in-all, I imagine the common elven population is overwhelmingly the equivalent of 25 or younger, with a small proportion of "adults", and vanishingly small numbers of 65+ equivalent elves. Runey has said the elven population was in the "tens of millions" when humans arrived, and is now in about a billion. Even if we assume "tens of millions" means "100 million", and that not a single elf has died in the last 300 years (which is a pretty massive assumption), then you're still looking at a population that's 90%+ the equivalent of 40 or less. It also means, by the way, that their population growth has been substantial, and from a vastly smaller number of potential parents; until fairly recently, only those original "tens of millions" of elves were old enough to breed, and their population still increased by at least a full order of magnitude. The only logical conclusion is that elven birth rates must be higher than human ones, or else the population numbers are literally impossible.
The population of Earth has increased by an order of magnitude in the last three centuries, but that's 10-15 human generations. HH elves have increased by the same order of magnitude (or possibly even more) and that's with, at most, 2 or 3 common elven generations.