Weapons:
The whole "metal is worth 100 x normal" makes basically no sense, unless you're going to make metal arms and armor way, way better than they currently are. Last time I played dark sun, not one member of the party had any interest in keeping any metal item we found that wasn't magic, simply because selling them made us ridiculously rich, well beyond the benefits of those items. Looking at the rules you've got... I'm thinking it will turn out the same.
Races:
I don't see a problem with allowing any old race. The main reason not to is simply that rarity might lead to a campaign derailment by slavers.
Even without specifically mentioning the pristine tower, darksun was always presented as a setting where lots of mutations and weirdness exist, and you might find anything out in the desert.
I think it would do to have guidelines that cover starting campaigns in the various city-states. It never made sense to me that people could start as halflings in most cities, for instance.
Classes:
I feel like you're doing that thing 2e did where everyone has their class tattooed on their forehead. As long as you're clear where a character's power is coming from, then I have no problem with mix-and-matching classes and roles. Want a sorceror as a templar of a sorceror king? That makes perfect sense. Want a warlock as a psion? No problem. Want a monk as a defiler? No problem. Things just have to be reflavoured, I feel. You're not going to be able to pick typical animals for barbarian totem warriors or druid shapeshifts, but banning them seems harsh when other analogues exist.
Skills:
Making skills have perma-disadvantage doesn't work very well, because it removes disadvantage from the DM's toolbox when he's making a skill check harder.
I would suggest giving example DCs for history and survival. Arcana should probably just not be on most proficiency lists. Even most spell casters shouldn't have access to it.
Defiling:
I feel like your defiling is not worth using. It's the tiniest of bumps to power in exchange for quite a lot of loss.
I feel like preserving should be a choice that makes life much harder, because that makes it feel more like a temptation to defile, and it should be a fairly big loss to be preserving. Something like making one spell slot/day at each level a defiler-only slot would be more than enough I think. And yes, that means that if you only have 1 slot at your highest level, you need to defile to use it.
I feel like the attack cantrips don't really fit athas either.
Feats:
The restrictions here seem completely arbitrary, and run contrary to the assertion that "fighters lead athas' armies". Without inspiring leadership, there's absolutely no benefit that an army can get from a fighter.
Rules:
I personally don't like the gritty realism rules, because I think that doing literally nothing for a week to get spell slots back is silly. If your goal is to have overland travel gradually deplete resources, I think you'd be better off saying that long rests aren't possible without at least semi-permanent shelter and short rests aren't possible without shelter (ie - making a camp). This means that city and dungeon adventures don't have ridiculous pauses while the party rest for a week after mild exertion, but means that being out in the desert sucks hard. It also solves any problems you have with spellcasters nourishing the entire party in the desert.