Okay this was a while back but I'm going to add to this about some things.
Zalanthas being Mars - That's even worse than it might be...there is water on mars...FLOWING water at that...however...they estimate it to be about the equivalent of 38 Olympic sized swimming pools. Not very much. Now...this can be explained by saying a comet has struck the planet, perhaps on the far side of the planet for that matter. The point being a comet has a lot of ice (and other things) in it and this could have increased the total amount of water on Mars. (we don't know if there is water deep in the crust yet) So...making the assumptions that the water has been added to this planet via a comet or three on the far side of the planet. (might explain the near eternal dust storms and why there is dust up there constantly? Though that would probably be an extinction event but let's say it was moving Really Really Really slowly) Or there is a large stash of water below the planet. Without revealing to much, I have seen an incredibly large amount of water in game, a long long time ago. Think of it as a torrential river. So water is available.
Plants - evolution is the easy answer. Specifically, being capable of holding in their moisture much as a cactus or desert plant does, that's why they have that 'waxy' covering on them. Typically plants can have wide spread out roots, typically in a desert, to catch as much rainfall as occasionally comes, a big root ball to grab the water that is near the surface and pull nutrients it needs, or a taproot that digs straight down deeply. So perhaps an evolution that causes an amalgam of at least the spread out roots and the tap root, allowing the plant to reach deep to find some moisture, spread out to grab a little water when those occasional rains come through and are designed to hold it all in as much as possible. Super slow growth on top of this means it can survive a long time, a cactus is VERY slow growing, so too are these plants. The fruit bearing plants hold their water in their fruit, this doing two things, allowing them to have a supply on hand and also tempting animals to eat them and spread their seeds when they poo. Fruitless leafy plants have probably got a store below ground, probably deep along their tap root. That's why so many are poisonous, it's to protect their precious water, animals won't eat them because they would die, so these plants have evolved to having added/altered their water stores to be highly poisonous. So far, on Earth, if there is ANY water, no matter where you look, there is life. In science, unless we can prove otherwise, it is going to be held that if a place has water, then it has what is necessary to support life in some manner.
People - Evolution again, this time helped along with some hefty mutation. Similar things to as above but resulting in Elves, Evolved Humans, Dwarves...etc.
Insects - It would require lots and lots of oxygen. Also, as was mentioned a diminished gravity from Earths or insects that have an circulatory system similar to ours and the means to breath better than insects on Earth do. Evolution can do amazing things. But if we go with lots of oxygen and lower gravity, this could be supported by the whole Mars thing.
Oxygen - not really enough plant life to support replenishing the oxygen. At least not in the Known. I have always assumed that Northwards, a long ways away, life continues to flourish in some form, by that I mean plant life. If we look at the world from south to north, plant life increases in abundancy to the point of looking as parts of the savannah does in Africa. Meaning to the north, more plants, even further north perhaps even more plants. This would allow oxygen to spread.
But that could also mean more animal life up that way...and perhaps that is true. But it doesn't have to be great amounts of oxygen breathing life. Maybe in the great UnKnown out there, there is only small animals. Maybe we are just the super duper unlucky ones that live down here, stuck as it were, where things suck really really bad.
The Black Moon - a captured rogue 'planet' or moon? It happens. Well...it theoretically could happen. And in Mars' case, that's where Phobos and Deimos came from, capture asteroids basically. Our moon likely came from Very Very Very Old Earth being struck by something planet sized. Stuff got ejected out into orbit around these fusing planets which over time pulled together and formed our moon. But anyway...one thing I have not mentioned is magick. Now...in Dark Sun one of the reasons the planet sucks so bad where the game is set is all the magic is pulled from plant or sometimes animal life. This can often kill the plants and make the land unlivable. Which would also kind of explain why the south is crappy as Tektolnes lives there and the north isn't quite as crappy because the Sun King was different from Tektolnes in a lot of ways. That being said, perhaps there was a black, rogue planet coming our way and magick stopped it's acceleration into Z's gravity well and placed it in a stable orbit like the other moons.
Suk-Krath being red - Well...if we go with the Mars analogy, that's simple, it's the Red Supergiant that Sol will turn into one day. Probably increasing in size still, though who knows? Maybe it's way after that and it's collapsing now. However, if it was at it's peak it would be pretty darn massive alright. It's going to swallow the Earth most likely, or come really close to doing so at least when it happens. That's 8 light minutes. 89,420,000 miles in radius, meaning 178,840,000 miles in diameter and placing it at the minimum point of it's orbit only 33,900,000 miles from the surface of the sun. The average distance from Mars to Earth's orbit is about 140,000,000. I'm not going to do the math here, but the sun would be a WHOLE lot larger than it is now...and a lot closer to mars, meaning it would be relatively huge. Something like a quarter of the sky? Though I doubt it's at it's full expansion and like I mentioned, probably on the way to that or on the way back down as it cools. (cooling being a very relative thing when we are speaking of an enormous fusion explosion in the sky)
Otherwise it's a Red Dwarf star. (it's not just a show you know) And that would mean Z is on the very closest 'edge of the habitable zone. Which would mean the sun was maybe as large as Sol is from Earth though likely smaller. As a Red Dwarf is about...oh 60% of the size of Sol. Red Dwarves are cooler (again being a very relative term to stars, not to what humans would consider hot and cold) than Sol is and the Habitable Zone is therefore closer, and Z would be on the closer edge of that probably to account for the heat.
I say it has to be hot because of the closeness of the star because if it's because of greenhouse gasses in the atmosphere...well...there probably wouldn't also be enough oxygen to support the giant creatures and insects.
There is a very good point for how fast things would burn in an oxygen rich atmosphere though. So, we probably have to look at a less massive planet than planet Earth to explain insect sizes.
The hardest thing to explain is magick. Sorcerer magick isn't quite as hard as it pulls it's energy from elsewhere and transforms it into a different kind of energy. Energy conversion is simple to believe in because it happens to us all the time. Electricity into light and heat. Light into heat and electricity. So on. (any technology that is far advanced beyond what we know would appear as magick, walkie talkies in the Middle Ages for instance.) From the helpfiles: "Sorcerers do not 'automatically' gain energy to cast their spells as elementalists can. They must instead gather it from sources of life." So there defilers handily handled, we just haven't advanced to that level yet. (and may we never reach that or we will be living on Zalanthas here on Earth) The elementalists don't SEEM to pull their power from the world around them. So that gets a bit harder to start to explain but...I'm going to assume it is somewhat similar and I don't want to say to much on how to explain that. I don't know if that's a secret or not but if someone wants me to really talk about it, I suppose I can.
Spice - Well. It's probably from poop. Secretions from Silt Horrors and/or Silt Flyers and/or whatever else is out there that we haven't really seen yet. Their digestion perhaps requires a unique method and adds chemicals to their food to work, these chemicals are expelled, probably after a long digestion period in which all water is drawn back out of it, meaning it comes out more like dust than what we would typically call poop. So, dusty poop has strange chemicals in them that if human or human-like species snort/smoke gets them high. Not really all that less weird than licking frogs. Which is a thing.
As for the amount of available meat sources in the game - okay...well. I always looked at this one as more of a gameplay thing than a science thing because I haven't come up with a scientific explanation for it. So for gameplay reasons...there are VNPC people in the cities, I always thought of VNPC beasties in the waste. I mean...there kinda has to be. So...yeah. That's all I got on the food supply thing.
Yeah this is sort of a necro, but I love theorizing about science...and it's still on the front page of this forum sooooo....