Would you consider something like this as supporting the claim of plagiarism?
The second possible way the prophet's recitation (and written Qur’an) could have included a conflation of the two Syriac loan-words puqdana and purqana is to assume that the text was already written down during Muhammad’s lifetime... In this case, Muhammad himself may have been responsible for conflating the two words found in older religious texts on which he drew in compiling the Qur’an, or he may have copied the terms correctly and distinct from one another, but (as in the first scenario above) later copyists then confused them.
https://www.academia.edu/1013511/Quranic_Furqan
And I think you linked to this article earlier:
Sūrat al-Fīl is an evocative text despite its brevity.
It would have had considerable power in Late Antiquity, when recited to an Arabic-speaking audience as a poetic reference to the canonical (in Eastern Christianity) Biblical narratives of 2 and 3 Maccabees. Q 105 invokes and repeats the powerful theme of monotheistic devotion that brings salvation via divine punishment of the oppressing polytheists, the same theme which forms the core of 2 and 3 Maccabees, using the same striking motif of royal war elephants as both the symbol and embodiment of hostile pagan power.
With their shared emphasis on martyrdom, perseverance, and divine salvation, the books of 2 and 3 Maccabees present religious views that converge upon what later emerged as Qur’anic theology, and comport with the ecumenical, indeterminate monotheism that scholars have increasingly advanced as constituting the early Qur’anic historical context. As one of the most archaic Qur’anic compositions, Q 105
would have seized upon its audience’s existing knowledge regarding this scriptural tradition of shared Jewish and Christian salvation narratives, which almost any Arabic-speaking monotheist could recognize and heed as a divine message.
https://www.academia.edu/11493284/Maccabees_Not_Mecca_The_Biblical_Subtext_of_Sūrat_al-Fīl_Q_105_
The Quran relates to existing traditions, I think that it is problematic tell the difference between for example, a lectionary that references existing texts that were pervasive in the cultural environment and a plagiarism, which pretends that at least some unoriginal aspects of these texts were original work of Muhammed.
You have works of religious rhetoric that reference Biblical and para Biblical stories and religious mythology to make a theological point. For example "They will question thee concerning Dhool Karnain. Say: 'I will recite to you a mention of him." The story of Dhul, as you know, is based on the Syriac Alexander Legend, is used, in the Quran, to make a point about the end of days as well as a warning against shirk.
What we have is a work of religious rhetoric, which draws on a story that was well known in the cultural environment. To plagiarise this would mean trying to pass this off as an original composition, although given its pervasiveness due to its links to Heraclius' propaganda, is it not more likely that it is a reference to a story that was well known to the audience. This is how an orator would operate.
The question is though, which knowledge, in what way did Muhammed claim to be a conduit of God?
As a hypothetical, let's use the not implausible idea that Muhammed was an eschatological prophet speaking largely to a relatively ecumenical audience of purported monotheists.
The divine knowledge claimed could relate to knowledge of the eschaton, and Muhammed was a warner to people to revert to the true message of God before it was too late. Muhammed then created rhetorical orations used to persuade his audience of the importance of his message.
This is not what is claimed by the tradition, but at least some of the tradition is acknowledged to be of dubious historicity.
From the perspective of academic history, plagiarism would require not simply using existing traditions but false claims about the authorship of such traditions. What was claimed in real life about this is unknown though. It only works if you accept the accuracy of the Sirah/hadith, and in these Muhammed starts gaining magical powers that are likely to be later additions, as Jesus' miracles likely were.
If Muhammed was using stories that his audience knew were not of his composition, I don't see how the charge of plagiarism can fit. If talk of angels and caves is a later addition and hagiographical, which is at least a possibility, then it is not sufficient to demonstrate anything about the real man. If plagiarism is dependant on this aspect of the tradition, it only 'works' if you assume the tradition to be true an accurate reflection of what Muhammed claimed, which is certainly 'unproven'.
Which gets back to my point, which is from the view from academic history plagiarism charges rest on information that is likely ahistorical.
Terms such as intertextuality and hypertextuality are certainly relevant, and, in my opinion, far more useful than 'plagiarism', which requires additional assumption.
