The biggest hole I can find in your argument is that very clearly somebody conquered the region. We have plenty of evidence and descriptions from non-Muslim sources.
It is undeniable that an ARAB conquest occured in the early VIIth century, just after the end of the last Byzantine-Sassanid war.
What is far from clear is that this was a MUSLIM conquest.
I'm not saying that the narrative we get from Islamic history is totally true - religious origin stories tend to get mythologized after all. However, the general basis - with a Prophet, some successors, and a conquest, seems to fit with what happened. The biggest problem with the revisionist histories I've seen is that its easy to poke holes in the narrative, but difficult to supply your own that doesn't have similar or even greater holes of its own.
I disagree.
This an alternate narrative that I find way more convincing:
- After the Byzantine-Sassanid wars of the 610s-620s both old empires are exhausted.
- Arab Foederati (Lakhmids et al.) take advantage of this situation to gain autonomy first and then outright independence. This does not take place in an organized or coordinated fashion. Simply, local Arab dynasts in Syria and Irak just stop taking orders from their nominal overlord while not necessarily overtly challenging said overlords.
- A few decades of anarchy follow, during which Iran, whose central power structure has collapsed, is conquered piecemeal by various Arab groups.
- The same then happens to Egypt, which has been left completely unguarded (except by a few Arab foederati ...) after the end of the Byzantine-Sassanian war.
- In the 660s, one of the new regional Arab strong men, Muawiyah, embarks on a consolidation adventure and manages to subdue his most powerful rivals.
- In order to further cement his hold on power, Muawiyah lauches a raid on Constantinople in order to focus the military energies of the newly federated Arab tribes on an external goal instead on internecine struggles.
- Even if the siege of Constaninople does not succeed, Muawiyah's goal of uniting the Arabs is met and the Umayyad dynasty's legitimacy is consolidated. Further Arab military adventures are now directed to the West (Tunisia) and the East (Central Asia, India).
- Religiously, there is no "Islam" yet by this point; just a tolerant religious policy pursued by the early Umayyads in order to win the allegiance of their subjects who are mostly Monophysite and therefore resentful against Constantinople's earlier persecutions.
- In order to appear acceptable to as many people as possible, the Umayyads officially embrace a form of "minimal Monothism" enshrined in a series of texts in Arabic, some of which may predate the Arab conquest and come from various sources (Christian missionary tracts, heretic sectarian texts, etc.) These texts will later be collected in "the Quran" (i.e. "The lectionary") but they are still circulating independently at this point.
- Towards the end of the VIIth century, this "neutral monotheism" is enshrined in the building of the "Dome of the Rock" in Jerusalem which is then still considered the central location of all forms of Monotheism as there is no Mecca yet.
- In the 750s, after a century of Umayyad rule, a dissident religious movement originating in lower Mesopotamia and Khurasan finally overthrows the last Umayyad Caliph and installs the Abassids in their stead.
- Contrary to the vaguely Monotheist Umayyads, the Abbasids are a militant messianic movement. They are strongly ideological, with quasi-totalitarian tendencies. In order to legitimize their rule for good, they embark on a wholesale history-rewriting program, weaving various stories of local conquest into a coherent whole supposedly centered around a prophet who had preached a new religion a century and a half ago which is in fact the very creed the Abbassid sectarian backers have just invented.
This history-rewriting endeavor is made quite easy by the fact that the whole Mesopotamian-Iranian sphere has no tradition of history-writing comparable to what the Greeks and Romans, or even the Jews, have. It seems that the Sassanids, for example, had the haziest of notions about whom the Achaemenids were and when they encountered Achaemenid tombs or ruins, they generally attributed them to the legendary Kayanids. It is true that a strong historical tradition does exist in Syria and Egypt but these regions had just lost the latest power struggle as they were the power base of the defeated Umayyads. No one there was in any position to contradict the version of the victors. In any case, as modern examples show (Stalin, Mao, the Kims) history rewriting can even occur in modern, highly litterate, societies with rich historical traditions. Is it so difficult then to imagine it occuring in societies where the litterate represent a tiny fraction of the population?
One thing that the Abbassids are forced to keep is the Quran (by then cannonized) because it is already too well known and widespread to be rewritten. The end result is what we can still witness today: a religion based on a set of official "historical" texts which are highly inconsistent with the sacred text around which they are supposedly built. It is very difficult to reconcile the Aadith and the Sira (Muhammad's biographies) with the Quran. Even today, devout muslims are strongly discouraged from comparing the two and equally strongly encouraged to rely on the former rather than the latter for their spiritual and moral guidance. The Quran is suposedly the only revealed text of Islam. But few muslims understand or even read it. By contrast, every Muslim knows the rudiment of the Sira and looks up to the Aadith for everyday guidance. None of these texts predate the late VIIIth century. They are the direct product of the Abbassid era.
I am not certain that this version is 100% correct. Indeed, we will probably never now for sure. But it looks way more credible than the official version and it fits the known data. In particular, it is consistent with the fact that there is absolutely no trace of any Caliph before Muawiyah, while there are indeed traces (coins in particular) of various local Arab leaders, who are definitly not Caliphs, in Iran, Syria and Egypt in the 640s and 650s.