Much of Dark Sun is left with a very wide overview of the world. As such how the species came about, whether Orcs were corrupted elves (LoTR style) or merely another species alongside human, elves, dwarves etc. is very much up to the DM. Does the DM want to explore that with the players or keep to the defined, “orcs were wiped out by the champion”…DM decision. The events around Kled and Kemalock do a great job of demonstrating the origins of dwarves (paintings, statues etc showing dwarves did indeed wear heavy armour and …KEEP BEARDS).
As above comment, the final choice is up to the DM. Dwarves could be an offshoot of the more ancient race of halflings (personally a ridiculous idea but its just an option example).
As for whether you put Orcs into the setting, my personal view (and my choice for my Dark Sun) is a no, they have been wiped out. However, its always been clear to me that whilst the setting history has chosen to remove orcs, the Tarek is pretty much an orc. A species known as having a more combative culture, known for being tough and brutal in combat (I think I recall Rikus fighting with\or alongside, a Tarek and noted it as a tough opponent). What the setting doesn’t do, is specify that Tareks are inherently evil (as opposed to orcs in standard 5e). The Tarek is a tough species, liken to the ancient tribes of Europe, and like those tribes they were very resistent to outside control and as such the Mul was still a good choice (Muls were born into a controlled environment and effectively brainwashed into serving their masters).
All of this is of course the DMs choice. Some DMs of old did indeed make Dark Sun to have been the forgotten realms, sundered and twisted and Rajaat and the other Pyreen were Primordeal entities of god like power.
As far as we know I believe that all humanoid species are descendants of the original Rhulisti Nature Masters/Nature Benders created through the power of the Pristine Tower as part of the adaptation to the new Green Age. Most new species are considered to be generated by the strange mutagenic powers of the Pristine Tower
all humanoid species are descendants of the original Rhulisti Nature Masters/Nature Benders
Gith are the one exception that immediately springs to mind. Per (messy) canon, they’re the degenerate descendants of Githyanki that were stationed on Athas at some point ages ago.
Yes! Which ofc is how the original Rhulisti arrived as well, from the Astral, so it is totally conceivable for a DM to include offworlders. Isolated divergent versions of species are very cool