How is the basic process question answered? There are three parties, Allah, Gabriel and mo.
How do you decide who contributed what? Why?
And to further muddy the waters, you have verses which bestow the concept of ‘spoken infallibility’ on Muhammed (Surah Najim, 53: 3-4), even though he made a few well-documented claims which have subsequently been proven wrong or mistaken. He admitted their erroneous nature on the record.
These proven mistakes inescapably introduce active human agency, Ijtihad, to what should otherwise be Muhammad’s passive reception of the Qur’an.
Indeed, the very fact that very few surahs were revealed in complete and finished chapters, as we have them now in the Qur’an, must have allowed Muhammad organisational interference with what he was given. That such and such ayah should go with this or that surah is not a secret in tafsir, especially when Muhammad started dictating the Qu’ran during his life time in Medina.
Muhammad later said that he took instructions from Gabriel on such matters, although certain verses and hadith contradict this and Gabriel having had any leeway in his delivery services. More importantly, it is in surah Qyyama where an argument could be made that Gabriel was supplying clarifications (ثم إنّ علينا بيانه) to Muhammad which would practically denude the Qur'an from its literality claims from the project's point of origin.
If this chapterising is loosely editorialising, then one famous scribe and earliest Qur’an writer was Ibn Abi Sarh who renounced Islam and made pretty serious claims that he had deliberately been unfaithful in taking dictation from Muhammad.
This did not cause a lot of headache to either Muhammad or his companions because Muhammad, the illiterate, told them that Sarh was lying about lying. They, of course, took Muhammad’s word for it and when Sarh re-embraced Islam, he had to endure very angry Muhammad. Because of what Sarh did, he was one of eleven people on whom Muhammad issued death fatwas; kill these 11 people wherever you find them, even if you find them taking refuge inside the Ka'ba. This was breaching the sanctity of alharam (the Arabs' pre-Islamic Green Zone in Mecca where no killing took place; this was the ambassadorial extraterritoriality in modern times where defectors, dissidents and fugitives hole up.) which was a most serious sin, which Muhammad claimed its divine suspension for one day hour.
Staying a bit longer with the Sarh saga, Sarh was a milk brother to Uthman ibn Affan and had taken refuge in Uthman's house in Mecca where the two met during the early stages of the Conquest of Mecca. Uthman took his brother to Muhammad and pleaded with him to forgive him. Muhammad was silent and then accepted Sarh's pledge of allegiance. When Uthman and Sarh started walking away, Muhammad looked around and asked his companions "what stopped anyone of you from killing Sarh?"
Clearly Muhammad had not forgiven Sarh his writery transgression and it might well have been his considerations for Uthman and Uthman's intersession that stayed Muhammad's hand.
The Islamic consensus on Muhammad's spoken infallibility above divides his comments into utterances on religious matters and utterances on not religious matters. (Life can be a funny thing and you should not suppress a chuckle at this obviously secular division and state of affairs.)
Muhammad himself has made the case for his part-time infallibility when his interrogative question, about why the Madina people were pollinating palm trees, was misunderstood and taken as Muhammad forbidding them from doing so, maybe because, they must have assumed, a new agricultural policy were in the offing. However, the yield that year was remarkably low for these farmers. They wondered why, and when he understood the confusion, Muhammad announced and made his infallibility known regarding this non-religious matter.
Muhammad's infallibility in religious matters can be challenged by him shortening Zuhur prayer to two rakat (حديث عبدالله بن مالك بن بحينة) which some companions have taken as the latest directive from Brussels. I mean, from Muhammad. These companions have left the mosque after the prayer and it was sometime before Muhammad was told about his mistake. As he had not left his prayer place, Muhammad turned around and made two prostration of forgetfulness (Sujood Alsahoo
This obvious case for Muhammad making a mistake in matters of practising his faith is normally brushed aside because of Allah's legislative wisdom; that Muhammad make a mistake for his ummah to learn from it. Everything, including Muhammad's errors, is grist to the mill.
Muhammad's infallibility and perfect recall of the Qur'an is further opened to debate in these two particular verses [87:6-7] (سنقرئك فلا تنسى * إلا ما شاء الله). Traditional Islamic scholarship tends to dismiss this i.e. that Muhammad did forget a part or parts from the Qur'an; they tend to dismiss it and explain it away by considering the case for negation in the verse to mean " so you do not forget " and not " so you will not forget". This latter has been excluded by almufisroon because the La', if it were the so-called Forbidding La', then the Alif in (تنسى) would have been dropped from the rasm. Really?
How could they get themselves out of the authenticated hadith, in Sahih Bukhari and Sahih Muslim, that Muhammad heard a reciter in the mosque and saying "may Allah have mercy upon him, for he has reminded me of so-and-so verses which I had forgotten or mistakenly dropped"?
In addition, there's the case for Qur'anic Quira'at when a word's meaning changes in one recitation and realisation to the other. Such differences are not allophonic (i.e. accents in one language) but are phonemic (i.e. different meanings) and would add more to challenging the prevailing consensus around the singular origin of the Qur'an and the passive role of Muhammad in relaying it to the Ummah verbatim.
Finally, more could be examined and critically analysed using the most trusted and widely accepted Islamic primary sources to undermine the case for fallibility by showing that the case for it in the texts is not remotely as certain as the Qur'an, hadith and Islamic scholarship would have us believe.
Cue in the biggest danger imaginable to the whole project; doubt.