I personally have my doubts about a Nehekharan god being responsible because, IIRC, Nehekhara wasn't a desert they were actually ruling over a civilization there.
It was in some places, but definitely not as bad. It suffered extreme desertification as a result of Nagash killing everything, including the Flora. I think something involving Zorn is more likely, but there's clearly divine magic involved, and while there
are forests around Zorn in the heart of the Southlands, I doubt they're northern trees like we're seeing here, and it really doesn't explain Cor-Dum at all. I guess it's possible that they managed to establish some sort of magical connection or contact with Zorn and the thing with Cor-Dum being born to the hold happened? I really don't know, and the trouble is that an Elf connection doesn't satisfactorily explain the weird desert stuff, and a Zorn connection doesn't satisfactorily explain the forest and Beastmen, unless it's simply the result of a God breaking down the terrain of the Chaos wastes and reducing it to a fine, magically neutral sand. Also, Khsar exists, who is explicitly a desert God.
Does Nehekhara have a oasis god?
Because trees In the middle of an desert looks like an oasis, even if the middle is a mountain instead of a lake.
If there is, I don't know them, and I'm not terribly well versed in Nehekharan Gods. And I'm going to bold this because it's related to the
End Times which is probably not canon for this quest but, Morr and Usirian are strongly implied to be the same entity.
They do however have a desert God.
If Karak Zorn is older than the Karaz Ankor and the Karaz Ankor was founded by a Dwarven exodus lead by the Ancestor-Gods, wouldn't it make sense if the Dwarves of Karak Zorn (used to) have other gods that may well have a desert connection of some kind and be more liberal when it comes to divine intervention?
Very likely, yes. They were created by the Old Ones once upon a time, and they definitely predate the birth of the Ancestor Gods. Given that the Ancestors rose to prominence when the world went to hell, and Karak Zorn almost literally straddles the temporal border that marks the beginning of recorded Dwarfen history, as well as being the most Southernly hold which is remembered in any capacity whatsoever...
See this mountain spine that divides the continent?
Dwarfen history begins a lot nearer to the bottom, and from what little we know OOC, they chased mineral veins and established colonies moving
Northward. Zorn is the last hold in that range that anyone still remembers.