What if scholars can prove beyond reasonable doubt that the Koran was not dictated by the Archangel Gabriel to the Prophet Mohammad during the 7th century, but rather was redacted by later writers drawing on a variety of extant Christian and Jewish sources?

The first part of the sentence is already pretty clear : the Koran was put into writing 50 years after the death of Muhammad ; that is uncontroversial in most Islamic circles.

Not so clear, since the orthodox of version of the Qur'an generally accepted today was supposedly compiled by Zayd ibn Thabit and compiled by the 3rd caliph Uthman ibn Affan in about 650, i.e. only 18 years after Muhammad's death, and thus reasonably early enough to imagine that some supernatural being dictated divine proclamations to Muhammad who re-articulated them to his community whose oral tradition could have preserved the original words intact until they were standardized by Uthman.

However, the more important point is whether strong evidence can be put forth that makes clear to the majority of Muslims (that is, not just scholars and intellectuals) the fundamentally mundane origins of their holy text.

In fact, the "earliest Qur'an known to exist", the Sana'a manuscripts, discovered in Yemen in 1972, which were carbon-dated to 645-690 AD (according to that Wikipedia article quoting Carole Hillenbrand, The New Cambridge Medieval History, vol. 1, p.330), i.e. a couple of decades after Muhammed's death:

... some of these fragments revealed small but intriguing aberrations from the standard Koranic text [i.e. the one compiled by Uthman ibn Affan]. Such aberrations, though not surprising to textual historians, are troublingly at odds with the orthodox Muslim belief that the Koran as it has reached us today is quite simply the perfect, timeless, and unchanging Word of God.

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To date just two scholars have been granted extensive access to the Yemeni fragments: [Gerd-R.] Puin [a specialist in Arabic calligraphy and Koranic paleography based at Saarland University, in Saarbrücken, Germany] and his colleague H.-C. Graf von Bothmer, an Islamic-art historian also based at Saarland University. Puin and Von Bothmer have published only a few tantalizingly brief articles in scholarly publications on what they have discovered in the Yemeni fragments. They have been reluctant to publish partly because until recently they were more concerned with sorting and classifying the fragments than with systematically examining them, and partly because they felt that the Yemeni authorities, if they realized the possible implications of the discovery, might refuse them further access. Von Bothmer, however, in 1997 finished taking more than 35,000 microfilm pictures of the fragments, and has recently brought the pictures back to Germany. This means that soon Von Bothmer, Puin, and other scholars will finally have a chance to scrutinize the texts and to publish their findings freely--a prospect that thrills Puin. "So many Muslims have this belief that everything between the two covers of the Koran is just God's unaltered word," he says. "They like to quote the textual work that shows that the Bible has a history and did not fall straight out of the sky, but until now the Koran has been out of this discussion. The only way to break through this wall is to prove that the Koran has a history too. The Sana'a fragments will help us to do this."

What Is the Koran?", The Atlantic Monthly 1999 January

The Wall Street Journal article notes:

In the early 1980s, when the archive was still thought to be lost, two German scholars traveled to Yemen to examine and help restore a cache of ancient Quran manuscripts. They, too, took pictures. When they tried to get them out of Yemen, authorities seized them, says Gerd-Rüdiger Puin, one of the scholars. German diplomats finally persuaded Yemen to release most of the photos, he says.

Mr. Puin says the manuscripts suggested to him that the Quran "didn't just fall from heaven" but "has a history." When he said so publicly a decade ago, it stirred rage. "Please ensure that these scholars are not given further access to the documents," read one letter to the Yemen Times. "Allah, help us against our enemies."

With this huge corpus of text recently obtained from Sana'a and now the Pretzl/Spitaner archive being worked on by Angelika Neuwirth ("former pupil and protégée of the late Mr. Spitaler") It would be interesting to learn what Mohammed Arkoun would have to say regarding their impact on Muslims' view of the Qur'an:

Deviating from the orthodox interpretation of the Koran, says the Algerian Mohammed Arkoun, a professor emeritus of Islamic thought at the University of Paris, is "a very sensitive business" with major implications. "Millions and millions of people refer to the Koran daily to explain their actions and to justify their aspirations," Arkoun says. "This scale of reference is much larger than it has ever been before."

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Arkoun argued in Lectures du Coran (1982), for example, that "it is time [for Islam] to assume, along with all of the great cultural traditions, the modern risks of scientific knowledge," and suggested that "the problem of the divine authenticity of the Koran can serve to reactivate Islamic thought and engage it in the major debates of our age." Arkoun regrets the fact that most Muslims are unaware that a different conception of the Koran exists within their own historical tradition. What a re-examination of Islamic history offers Muslims, Arkoun and others argue, is an opportunity to challenge the Muslim orthodoxy from within, rather than having to rely on "hostile" outside sources. Arkoun, Abu Zaid, and others hope that this challenge might ultimately lead to nothing less than an Islamic renaissance.

What Is the Koran?", The Atlantic Monthly 1999 January