Hmm. I don't believe the match in detail between "Ubar" and Lovecraft's "Nameless City" was as exact as you think. (Heck, right there we have a significant mismatch -- if it was called "Ubar" it was hardly "nameless, was it?) What could be happening here is a classic example of circular storytelling.
In ancient times a city rises along the Frankincense trade routes called "Irem" or "Iram" in the region known as "Ubar." This city becomes wealthy but then disappears for some unknown reason (unknown to the colloquials, certainly), and is later discovered to be mostly or partially buried in the sand, with all the people fled.
Several hundred years later, Mohammed, the Prophet mentions the City ("Iram of the many pillars"
in his Quran. Most modern people reading the Quran are unfamiliar with the city and believe it to be made up entirely as a moral lesson -- and probably to be the root of the legends about a lost city in Southern Arabia.
Over the next several hundred years, various tales, tall and otherwise, from the vast realm of Islam are accumulated in a book known variously as the "1001 Nights," or "The Thousand Nights," or "The Thousand Nights and a Night." Among these tales is one called "The City of Brass" which recounts stories of a lost city in the desert of southern Arabia -- stories that, like all the others in the book, have floated around Arabia for centuries....
HPL reads
The 1001 Nights (and becomes literally enthralled by it, memorizing vast sections of it and even creating a false persona for himself -- coincidentally named "Abdul Alhazred"
.
Later he decides to write a story entitled "The Nameless City" which he describes in terms similar to those found in a story in
The 1001 Nights called "The City of Brass."
1400 years after Mohammed mentioned it in the Quran, a lost city in Syria, Ebla by name, is excavated and numerous account tablets found therein list the city of Iram as a trading partner. Suddenly there is a real possibility that the city actually existed.
In the 1980s a group of scientists, fascinated by the possibility such a city could exist, use satellite imagery to discover ancient camel caravan routes in southern Arabia, determining where they intersect (which is a good indication there was water in that location, and which in turn might indicate a city or town was built there). Using this imagery they eventually explore a site in Yemen (on the southern end of the Arabian peninsula) which they discover to have at least been a powerful fort thanks to a vast, naturally occurring limestone cavern which served to store large quantities of water. Overuse of this water led to the limestone drying out, becoming brittle, and eventually collapsing the city or fort onto itself and subsiding into the cavern -- thus burying the wells and water for centuries, and, coincidentally giving rise to legends of a "city" swallowed by the sands of the desert -- doubtless in response to some hideous sin, as later recounted by Mohammed in the Quran!
Thus, we see an actual city/town/fort that suffered an unexpected, but natural, disaster, pass into the legends of a region, subsequently picked up by a religious zealot who uses it as an example of what a lack of piety can cause, and which eventually becomes so much a "story" that the whole legend is considered apocryphal by modern scholars (who only find mention of it in a religious text and a book of fairy tales), until they suddenly discover that the city mentioned in the religious book was quite real, and then find a locality that both seems to match the description and offers a logical explanation of how the legend began....
Does that make the Necronomicon real? Alas, in all of this there is no mention of the book, except by Lovecraft. All in all, I'd say you won't find a "real" Necronomicon (or Al-Azif, either) anytime soon. But then, perhaps I've got it all wrong, and Nyarlathotep DID destroy Irem of the Many Pillars and cast the City of Brass into the sands....
H.P. Lovecraft 
