I've never until now heard of any particular reason to dispute the historical consensus I've always encountered that Islam and the organized Arab eruption from Arabia occurred essentially as the Koranic/Abbasid accounts would have it. Having been required to attempt to read the Koran within a week (might have been able to do that for myself, perhaps, but I was reading it aloud to a disabled partner, so I had to skim) I found its textual character reasonably consistent with writing down what one individual prophet was saying, and elements of it seemed consistent with a personal, historical locus of political events. Anyway it is only now that I'm hearing of this radical revisionism that the whole thing was collated and historically re-written out of a very different sheaf of events and records.
In the latter case, I'd think that all along, throughout the entire Islamic era, there would have been a vigorous counter-argument denying the historicity of the Koran and all traditions associated with it. Hostile Christians, especially in Europe of course, would have picked up on this and collated a counter-narrative which would become the norm of European historiography. Even after the fires of inter-religious conflict had largely died out and relations with Muslim powers became expedient (as indeed specific western Catholic regimes did find expedient with specific Muslim ones from very early contacts on) at most this would result in a controversy of two narratives, with powers supporting good relations with this or that Muslim polity advocating for the OTL conventional one, but there would always be a refuge, indeed more likely the dominant consensus among most Christian powers, for the one that debunks the authority or even existence of Mohammed.
And yet, even when Christian writers were quite hostile to Islam and keen to adopt all sorts of beliefs about Islamic practices that were pretty much fanciful, they never did settle on a credible counter-narrative such as you (fi11222, the author of this thread and proponent of the radical revisionist alternative in Evilprodigy's thread) suggest is the actual historical basis of modern Islam. Lots of anti-Muslim polemics emerging from the Roman sphere were quite irresponsible and fantastic of course; they hardly needed a well-grounded critique when wild defamation would do just as well.
But I would think if there were such a huge gap between historic events and a canonized narrative collated centuries later as you suggest, that more or less responsible (if biased) Christian scholars in a Europe pitted in existential struggle with Islamic powers would, some time between the year 1000 and 1500 (CE of course! Or in this context, appropriately enough, AD) get wind of dissenting narratives in the Muslim world and its peripheries and collate your counter-narrative, adducing evidence from diverse sources, and develop a much more solid, historically grounded debunking of Muslim ideology--complementary with Catholic theology of course. Protestants might quarrel with details but would accept the basic thesis as well, Enlightenment era and 19th century scholarship would elaborate on it. It would not always, in every European power, be expedient to emphasize this, but there would hardly be a basis for some sort of Orwellian expulsion of all historic memory of this counter-narrative from the European historic tradition. On the whole I think it would obviously dominate the conventional view of Islam in the European west, and be used as justification for various attempts to extirpate Islam completely, in the name either of Christ or of secular humanism. After all the latter tendency did not balk at undermining the Christian consensus in its own heartland, why fail to denounce the other great Abrahamic tradition along with Christianity?
After all, it would not only be Christendom arrayed against the Abassid fabrications. However powerful that Caliphate might have been, surely it never reached full control over the whole sweep of the Islamic world as known OTL. They never controlled the far Maghreb; why would the Moroccans accept the narrative, doubtless coming to them belatedly after divergent precursors had reached them first?
By and large, legendary accounts typically turn out to have some truth to them. Not always; the effort to try and ground the Hebrew Book of Exodus in solid historic context seems to be chimeric for instance. But plenty of other Hebrew legends as recorded in say Genesis turn out to have some independent historic confirmation. The city of Troy turns out to have actually existed, and I believe archaeologists have come upon a consensus as to which sacking of the layered ancient city does correspond roughly to the events described in Homer. By and large, legend does turn out to have some correspondence to ascertainable fact.
So if events went as you describe, I'd think the Islamic world would be shattered into dozens of rival sects, just as Christendom was. Imperial Roman power did succeed in imposing Chalcedonian Orthodoxy on the various peoples who would, after Islam overrode and overwrote many of the dissenters, in their divisions between Eastern and Roman orthodoxy and then Protestant dissenters as well, comprise the vast majority of surviving numbers of Christians. But the historic fact of the various non-Chalcedonian dissents was never forgotten even so, and of course millions of Christians to this day maintain some trace of these once-numerous alternate rites and traditions. The fact that the Germanic conquerors of the Western Empire were not "orthodox" Catholics but Arians is of great and unforgotten historic importance. So, if Islam in its modern orthodox form was in fact a late imperial construction, I'd think that the nature of dissent in the Muslim sphere today would have a similar fragmented mosaic nature; we'd all know all about dozens of divergent traditions. Compared to the breadth and depth of the splits in Christian tradition, the split between Sunni and Shi'ite branches of Islam seems pretty slight and superficial, and would require that the dissenters who became and remained Shi'ites failed to remember and cite the fabrications of their Sunni rivals-or vice versa of course!
So to me it seems evident that the best way to account for the unanimity with which the Islamic traditions all affirm a large corpus of basic truths, despite their bitter quarrels over relatively small divergences, and that Islam's rivals have not long ago settled on a debunking consensus such as you offer, is to suppose that events of the Koran and the related Hadith and other core traditions of Islam were based firmly on historic fact. That Mohammed was an individual of great intellect and moral fervor who personally learned of and collated many strands of Jewish and Christian tradition and refashioned them into a coherent world view that corresponded well with the sentiments, beliefs and experiences of his Arab neighbors, and with prophetic conviction tapping into shared resentments and passions of his people, assailed the existing class structure in Mecca, was driven into exile in Medina, and there gathered up forces to return to Mecca and take it, and from this base and with expanding credibility, organized the majority of Arabs within range for collective action.
Since I am no Muslim myself, I don't suppose the clearly miraculous events of the Koran, such as his being bodily taken up into Heaven to receive yet more detailed instruction and inspiration there, literally happened, though it seems plausible enough that he had subjective experiences that seemed best understood that way to himself, and his interpretation was literally accepted by his followers. But on the whole, what reason is there to doubt that the secular events as traditionally attributed did happen more or less as described?
These events being fundamentally true and widely known, there would then be little basis for dissenters to base their differences on arguing their falsehood. The Christian critique of Islam would not find the fertile ground of widespread doubt about the historical narrative of events, only arguments about their interpretation.
This accounts well for the state of things as known today, including the entire European history of attacks on Islam.
I don't know what accounts for your out of left field suggestion that the whole thing is one huge fabrication instead. Certainly there are suggestions that Christianity is such a narrative re-collation of ahistorical legends and assertions, but Christianity also shows the fault lines that might lend some credibility to that kind of debunking. Islam seems much more historically solid, and I don't believe that any state of a thousand years ago could have the Orwellian power to so completely cover its tracks as to pass off a fabrication as reality. Even the development of the Confucian narrative in China could not completely paper over the divergent foundations on which it rested, and that was with the help of a tightly centralized state. Could the Abassids have exercised the sort of control over the far reaches of Islamic proselytization that the Han Emperors did over relatively isolated China? Even if they had, the fault lines would still be there to find.
So I'm afraid that your counter-narrative looks to me like the recent fabrication of people who are perhaps frightened of a possible Muslim ascendency and are looking for ways to try to undermine it, and observing the manner in which Western scholarship and perhaps partisanship have undermined the historic basis of Christianity, have sought to reconstruct a plausible sequence of events that would try to account for Islam as we know it today in terms of a collation of diverse narratives for political purposes, which is just how Chalecedonian Christianity can be described. But where is the messy detritus that would sprawl all around such a construction? And where is the caustic Christian critique that would surely have capitalized on such dubious foundations?
Tit for tat might seem fair to desperate people, but it is hardly scientific. As a non-Muslim I will readily concede that certain articles of Islamic faith are clearly false, such as the assertion, embedded in the Koran itself, that that document was produced by divine inspiration enabling the illiterate Mohammmed to personally scribe the whole thing end to end (in his spare time between all the activities attributed to him). But that it is a collation of things that he himself largely said, written down by faithful followers concerned to get it literally correct, and tied together and published by people who followed him with a close consensus as to what he actually said and did, seems pretty likely the case, probably with a lot of stuff taken wholesale from other sources--but with Mohammed's personal approval. I can accept that scholarship might prove that this or that passage or section was interpolated late, or that others once considered canonical were omitted, and that some creative editing might have happened under the early caliphs. But by and large, the unity of the Islamic world, the lack of schisms over the texts, all suggest to me these things progressed on a more unified and consensual basis than the development of Christian canon. The two cases are not in fact comparable. This obviously to me is no evidence that one was divinely inspired and approved and the other was not; I'd attribute instead to the supposition that Mohammed was of his people and hit a strong vein of resentment and reformism that was close to their collective hearts, and therefore there was much consensus among numerous people about his words and actions.