Just to let you know, I'm playing the devil's advocate here to try and help you develop a stronger, more robust world. It's clearly an interesting idea. I'm just trying to pick apart pieces of it to help you define it more clearly and to give your players less things to stretch their suspension of disbelief.
[*]Humans have a BIG brother. There's a god of humans, a curious property to human blood or human dwellings, or ancient magics in place since the Dawn Times which render human habitats inhospitable to the critters of the night. In fact, this one is reflected in Sigil, where the Lady of Pain keeps the peace and stops all the CR 8+ outsiders from eating the low level PCs alive. Think about it.
You could twist this a bit and use the holy symbol idea. As long as the human believes in BIG brother, they can keep the various nasty races back by displaying their symbol, because brandishing it causes pain and/or damage and/or weakness. To counter this, count on the nasty races developing various long ranged attacks. (Vampires don't like holy symbols, but vampires with crossbows or longbows at max range are maybe not so bothered.)
If you have an active big brother who is not a god, they'd have to be everywhere, in every human settlement. That or they'd be on all the border areas and some in the inner areas of human settlement, but it's understood that if you come in and kill some humans, big brother slams back hard.
[*]Humans are stronger than you think. PCs, far from being rare, are relatively common -- all werewolves are ferocious balls of fur, but every city has (in addition to somewhat useless men at arms) a full retinue of NPC heroes whose full time job is remaining in town to enforce the peace. What's rare are roving PCs; as they get to be higher and higher level, the remaining strongholds become more and more desperate to have them "retire", so that they can help keep the citadels safe.
This means that the men-at-arms will not be anything except human police. If a typical man at arms is puppy chow when a werewolf comes calling, then men-at-arms would not be trying to stop them and would be a completely separate organization.
For a real world analogy, when someone attacks your town with a tank, you don't send in the cops, you call out the Army.
A societal situation I think this would bring about would be in women trying to get pregnant from the male adventurers. This would be both to try and get the adventurer to stay in town and to try and sire more powerful citizens to help protect the next generation. (There is historical precedent for the idea of trying to breed powerful men to produce more powerful men.) Since the typical D&D world is far less gender biased than history was, you're going to run into more female adventurers and such. This would adjust things a bit, but I think what I've stated would still be true.
(To anyone who's offended by the above, please understand I'm only pointing out how our own world has worked in history, not advocating a particular mindset.)
[*]Human Reservations. Humanity is a relatively unmagical race, and so it's not as affected by dead- and dying- magic zones or wild magic zones -- and so they make their homes there. The other species don't want that land or have to interact with humans on pretty much the same level while on them, and so again, humanity is able to eke out an existence. This is an especially neat option because it means you don't have to think too hard about the repercussions of what you're designing -- on human lands, physics works the way we expect, so castles make sense. Elsewhere, magic holds sway, so +5 swords and dungeons make perfect sense.
This doesn't prevent raids into the human lands, it just makes them less effective. It does mean that the raids are without consequence, however, as the humans would be mulch if they tried to raid back out of the anti-magic reservation.
The other flaw I see in this is that slavers can still go into the reservation and extract humans, then bring them back out where they're easy to keep enslaved to their powerful master. If our own world's history is any indication, there will be humans willing to bring the potential slaves out and sell them to the mystical races. (For instance, the people of Africa selling slaves that were then bound for America.)
Historically, the strong enslave the weak. Why should a vampire till the soil when they can get a lesser race to do it for them? (To a vampire, a human is a lesser race.)
Economics are your answer if you don't want slavery to happen. Slavery is only viable if land is cheap but labor is expensive. This is why it was economically viable in the American south when there was tons of land but not enough workers. If land is scarce but labor is cheap, slavery tends to be far less viable. (I could go into more detail, but that generality will help you set up something reasonable.)
I believe you need to spend some time developing the history of the various races and their countries, at least in broad strokes. How did various areas come to be settled? Why are the dividing lines where they are? In general, where are the conflicts arising?
