Here is what Sinai wrote: “The conventional placement of the genesis of the Qurʾan in todays Mecca and Medina is by no means beyond reasonable doubt” (for the quote: The Qurʾan: A Historical Critical Introduction, Edinburgh, 2017, p. 59; for the entire discussion: pp. 40–77).
I do not have the book, so I cannot really see... If he feels obliged to say this kind of things the way he does, i.e, (for me...) not so clearly, it is because he knows very well that "Mecca/Medina/Kaba" is now historically untenable. That was not the case 20 or 30 years ago. But he displays that he believes that "prophet Muhammad" have existed because it is "clear that we have all the reasons to believe that he existed (!)" (see his podcasts : https://podcasts.ox.ac.uk/series/introducing-quran
Why not? But then the question is : where? Question to which he (of course) does not answer because he knows very well that what recount the narrative is improbable in Late Antiquity because that none allusions exist anywhere about a man to whom God is talking during 20+ years. On Mars, yes. On Earth not. The supposed "Mars" (Mecca) in Western Arabia, far far away from any influential religious people, constantly affirmed by the narrative because (of course) it would undermine the affirmation of the "prophecy", has never existed, and he knows it. Then to the question: "where then?", is necessarily responded, due to the geography, that it is with the religious people that the narrative is trying to get away from them to not undermine the "prophetical" contact with God.
Sinai argues for an Arabian provenance for the Quranic corpus (convincingly, I might add, at least for parts of the corpus), but, and as the above quote inform us, he is open to an extra-Arabian origins as well. .
Maybe, but I note that he seems not really interested to address this extra-Arabian origins historically because it is dangerous (and he knows it...). That is why academic scholars are trapped, as they are public people, they cannot really do their jobs freely. They risk their lives.
It seems to me that parts of the Quran that reflect an Arabian context might have indeed originated somewhere in Western Arabia and not in Syro-Palestine (on this, see Sinai, Qurʾan, 40–77; and also Süleyman Dost’s doctoral dissertation). This is unlike the parts displaying a strong Christian context.
What lies under this Sinai argument is that he'd seem want to make believe that it is not possible for someone to write something (reflecting an Arabian context) outside his own culture and that what is written in a certain language must come necessarily from someone of the ethnicity (therefore geography, therefore here peninsula "Arabia") to which belong this language. Both assertions are scientifically nonsense. It is not because what is describe in a text about religious practices of Arabs (Sinai podcast #2) this means that the writer of this description is Arab. It is yet what Sinai is implying implicitly whereas it is inexact
Moreover, I'm still waiting to someone to tell me what new specific "Arabian context" or new information about Arabs that was not available in the 6 or 7th c. to the Orient people about Arabs, the Quranic corpus brings to us which would prove that the Quranic text is perforce coming from persons of Arab ethnicity (therefore geography, therefore here peninsula "Arabia")? There is nothing. Nada. Therefore, considering this, I'm sceptic to the affirmation that the fact of reflecting an "Arabian context" or descriptions of religious practices means necessarily Arabs genuine origin. I have no evidence of that.
As for Dye, he thinks that the consonantal skeleton (rasm) of the Quran reached closure somewhere in the second half of the seventh century—that is, between 650 and 705 AD. This is unlike Sinai, who thinks it reached closure around 650 and that there are no further addition after that. Sinai also defends this position in his book as well. His arguments are the same (albeit not as detailed) as those presented in his two part article on this very topic.
Yes. Sinai does not believe in the Abd al Malik stuff.