No one disputes the fact that Islam existed
long before the official version of the Koran was written.
(I would even make the hypothesis that it predated the birth of the alleged Mohammed).
That is not the point.
My understanding of Wansbrough's work is
that it is based on a very careful textual analysis of the Koran.
The literary devices used in it were common
in the Syrian Christian and Jewish communities.
They would have been meaningless to the tribesmen of Arabia.
The entire Koran assumes the reader (or listener) was thoroughly familiar
with Christian and Jewish legends
which the Arabian tribesmen would have known nothing about.
Thus if there was a historical Mohammed who received a message from Gabriel
(and this certainly is one of the most central doctrines of Islam)
whatever this message was
it certainly was totally falsified in the official Koran.
But if this message was so totally falsified in later years
what then is there left to believe?
Perhaps there was no such message in the first place.
Perhaps there was not even such a Mohammed.
Of course Gabriel did not dictate the Koran to Muhammad. Not because there was no Muhammad, but because there was and is no Gabriel. It is a fairy tale. Santa Claus doesn't exist either. This is not news. The Koran as a book would have been "meaningless" to the tribesmen of Arabia because it was written and they couldn't read. Every serious scholar has known both as long as modern scholarship about Islam has existed, certainly since the 19th century.
You conceed the existence of Islam as the political entity it was, long before the codification of the written Koran. You have to, it is plain as a pikestaff. Yet you still want to insinuate in your closing "perhaps" that Muhammad did not exist. Um, who were all of those people who conquered the near east following and obeying and referring to in their internal legitimations and disputes? It wasn't Muhammad, of course, since he didn't exist - it was all made up about this other guy - named Muhammad.
Naturally, a whole army, state, and empire hit upon the bright idea of all agreeing they were following the orders given to them by a non-existent personage they all knew hadn't really been still walking around a handful of years previously. When they fought each other, they all agreed to do so by claiming legitimacy from this non-existent personage. The Armenian history of the events, written within living memory of them happening, was just as deluded as all of them about the existence of Muhammad, the message he preached, and its unifying political effect on them and its launching them on their conquests. It was a sort of collective psychosis, over the whole middle east.
Later, some Syrian monks cleverly put words into the mouth of the non-existent personage the others were all following, but just neglected to include little details like the Incarnation, probably because they overlooked it. Darn, if only they had included a little more in the way of traditional Syrian literary devices, Islam would be a branch of eastern Orthodoxy to this day.
The reason the whole new army and state could not be made Christian by inserting two stories into a batch is that they already weren't Christians and would not have put up with it. They already knew what they believed, and did not need scribes to write it down for them first. Possession of writing does not confer automatic ability to rule all of the thoughts of all illiterates.
Also, just a minor little point, they all agreed on what they believed, to an amazing extent. It was one of the great coincidences of history. They all said they thought these things because this guy named Muhammad - who was of course a figment of their imaginations - told them what to believe and they all followed his orders. The Armenians even knew what those were. But of course they just believed what they heard about it from delusional Arabs.
There is no getting around the connection between the worldly teacher, the basic doctrine, and the political movement. You don't get the effect without the cause. It is not a matter of a mythical fairy tale cause, but a real human historical one that anybody with half a brain can verify. New states all saying they are following the teachings of one man imply the existence of that man. Uniform consistency of their doctrine in determined political hostility to neighboring contemporary doctrines implies an innovating new source of a difference in doctrine.
Muhammad existed, he told his followers he was inspired, and preached to them a simplified monotheism based on what he said was the God of Abraham. It was a doctrine determinately different from those of the nearby great powers of his day, Sassanid Persia's Zorastrianism and Byzantine Christianity, borrowing elements from both. His followers took control of a small section of southwest Arabia, and from it as a base intervened in the ongoing power struggle between Persia and Byzantium. They conquered Persia, greater Syria, and Egypt, in less than 10 years. Syria became the developed heartland of the conquered territories. A power struggle for control of the new area was one by Syrian provincials, who moved the capital from its original seat in southwest Arabia to Damascus.
*Then* a lot of literary stuff happened. All of it long after the main religious and political events had already ben settled. The fact that the new governors of the new state believed definite things because Muhammad had taught them those things *constrained* whatever literary activity they put their own names to. Written Arabic literature appeared for the first time, as the conquerors learned literacy from their conquered subjects. Who were Syrian Christians - especially those from sects previously persecuted under Byzantium, like Nestorians and Monophysites, as more reliable than the Orthodox, who had a greater loyalty to Byzantium.
Then they had an additional political fight over the influence of Persian soldiery in an army previously Arab, along with innovations in doctrine based to a large extent on differences between what they had believed when they rode out of Arabia, and what the learned among them had been absorbing from Syrian theology - e.g. in disputes about free will, the problem of evil, etc.
The innovators won, backed by Persians, and moved the capital east to Baghdad (which had grown up next to Persian Ctseiphon as the Arab army garrison next door). The first kings of the new dynasty patronized all forms of literature, from translations of Greek philosophy and Indian religious texts - to large codifications of law. Official texts of Koran and of masses of hadith, as collections of material spanning the entire movement up to that point, were codified and published.
That history relates what every serious scholar has believed for a century. Not one line of it is contradicted by anything you've said. Every circumstance that either you or I have related fits into it perfectly, without anything amazing sticking out this way or that.
The broad outlines of the new doctrine and the person who preached it came first, and were not made up. Causes precede effects. The military-political earthquake that established the new thing came after that, hard on its heels, in a very compressed frame of time. It is recorded by a contemporary non-Arabic source, including referrence to the originator and the basic doctrine.
The place of Syriac literary influence on the whole development is also clear. It is -not- back before the early 7th century political events as their cause. It -is- before the final codified texts in Baghdad in the 8th century. The political timeline accords with the place of these influences. They occur -after- the new thing -conquers- Syria.
The reason the eventual doctrines are not exactly the same as Syrian Christianity is also explained - the Syrian originators of the literary conventions were -subjects- of new rulers who already believed a -different- basic doctrine. The reason for literary influence anyway, despite the scribes being subjects rather than rulers, is also explained - the rulers had no learned literary tradition of their own, because they were largely -illiterate- when they arrived. They were nomad soldiers, not urban scribes.
And the reason for the codification of the doctrine in textual form, its "settling down", is also explained. The political revolution from the Omayyads to the Abassids included the replacement of an unlimited exclusively Arab military monarchy interested primarily in power and loot, with a more law-limited cosmopolitian monarchy that patronized all forms of literary activity as a means of legitimation and especially of assimilation of vast conquered areas diverse in native doctrine.
Duncan McDonald (to pick one example of traditional orientalist scholarship) knew about as much a hundred years ago. None of the revisionist scholarship you have cited changes a particle of the picture of the events he already had. You don't get "credit" for not believing in Santa Claus; it does not mean that your guesses as to the existence of Karl Marx must be taken more seriously.