"Koran as we know it was gradually pieced together,"
True, known by all serious scholars
"he saw this process of piecing it together, the formation of a Muslim Scripture, of part and parcel of the emergence of Islam"
False, and even absurd as a matter of history, sociology, politics, etc. It conflates writing with religion and theological minutae (the content of this or that minor point of doctrine settled later) with massive political facts.
Islam existed as the creed of an army, as "regimental orders", long before any literate culture existed among its members. It is ridiculous to maintain that it did not exist until there was one accepted and bound Koran. Religions are not books.
It is far sillier even than saying Christianity began in 200 AD, because in that case at least its adherents were literate from an early period, and as a sect it had no political existence until much later. Whereas Islam was first of all one of the states on the map - an area dominated by an army, which obeyed one line of rulers.
It is crazy to say it "didn't yet exist" when it conquered the whole near east. Only a book about it didn't yet exist. Non-existent things do not destroy whole empires. "But it wasn't yet Islam" - sophistical nonsense, a verbal slight of hand, pretending that Islam is defined as a book. Of course it was Islam. Otherwise you are reduced to saying, "it was instead 'gefnord', which strangely enough looks exactly like Islam in every historical particular, but we just don't call it that until there is a Koran".
Incidentally, an English translation of Seobos' history, written in classical Armenian in the 7th century, only came out in 1979. Anyone without any knowledge of Armenian might easily have missed it as evidence, in these revisionists' heyday in the 1970s. Their model of what was going on would have predicted no such passage, nor any mention of Muhammad. It is a forced, tendentious idea pushed to the breaking point, and broken as a matter of scholarship.
What is true and indeed has been known to the whole serious scholarly consensus for a century, is that the Koran was written late, and the early period from Muhammad's own origin to the expansion of young Islam out of Arabia under Omar in 635 AD, has been subject to considerable revision, backdating, and wholesale invention by later Arab sources. Because it was a legal precedent matter for an important new state - nobody would have cared otherwise.
One can also see how nonsensical the view is on the internal doctrine side by following the actual doctrinal splits within Islam. Khajarites, Shia, and Qadarites all predate his supposed "origin". So does the shift to Baghdad and founding of the Abassids, and the Mutazilite theology associated with that event. Which is where Christian theology probably had its largest effect, actually.
The greatest period of "syncretism", shifting existing doctrine in response to theological argument taken from Christian sources, occurs around 750 in Baghdad (when the doctrine of free will is taught and enforced by a persecution), not 680 in Damascus. From the whole course of the fight over that doctrine, it is clear it was new, and that under the Omayyads fatalism was the accepted position.
Meanwhile, other events like the persecution of the Syrian church under Patriarch Mar Gewangis I around 670 (for refusal of tribute), recorded in Syriac sources not just Arabic ones, makes no sense if they were supposedly not Muslim "yet".
The more it is pushed, the more historical anomolies "stick out" and jar, making no historical sense. These guys want to push as far as they can in one direction anyway, except where constrained by a known non-Arabic text. When they know perfectly well it was an illiterate group, that almost everything was initially oral, and that there was essentially no interaction with non-Arab literate groups until after 635.
That is not the way historical scholarship is done. You don't get to write whatever fantasy you like in any place that looks "dark" in the sources, nor do you get to artificially increase the darkness to have more scope to do so, by rejecting as many sources as possible as supposedly untrustworthy.
Whatever story is told about early Islam has to fit the known facts, including e.g. the political divisions that already occurred, narrative of important and highly public political events like changes of rulers, assassinations, succession crises, major battles, etc. You can't falsify the very existence of such things close to the time they occurred. E.g. if every Arabic source on every side of the fight dates the Shia-Sunni split to a succession crisis after the fourth caliph, there is no reason to discount it.
Islamic *literalism* as an aggressive doctrine position dates to around the 750 AD period. We can name the man involved and the political events surrounding his taking the position (Ibn-Hanbal, persecuted by the Mutazilites of Baghdad). The codification of Hadith is somewhat later still. Islamic *literature* "settles down" by around 800 AD. But the political thing, and the main points of its doctrine, are all there by 645 AD at the latest.
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.