You haven’t accounted for the obvious breaks in the chain of transmission from Mohammed’s tales and fables to Uthman’s eventual writing / re-writing of the Koran.
Let’s understand that the lineage of the Koran is irreparably broken in three places. Those places are:
1. The ‘Uthmanic rescension.
2. The Compilation under Abu Bakr.
3. The chain of transmittal prior to Muhammad.
The fact remains that “Qur’an” means “recitation,” not book. It did not become a book until years after Muhammad’s death.
it seems that you depend in your argument on quoting the writings of the European orientalists. These writings are not confident for the following reasons:
1) orientalists were biased against islam and Quran, they were affected by their aversion toward muslims due to known historical reasons.
2) the methodology of orientalists is inferior to the islamic methodology regarding historical verification of events.
Orientalists based their methodology on written manuscripts. while islamic methodology is based on:
a) confidentiality of oral or written information is based on the confidentiality of its (teller). muslims recorded information about the private history of thousands of people to be able to judge their confidentiality. they prepared a ranking system for the people (strong / weak / liar). there are many details regarding this issue.
b) muslims used (التواتر) which may be translated as consensus as a proof for confidence. its meaning is that sufficiently many confidential people are telling the same information.
so what you describe as (break) is in fact a highly confident (information transmission) process.
the preservation of Quran proceeded in the following stages:
1) it was written immediately during the life of the messenger (peace be upon him), besides written manuscripts sahaba (early muslims) were reciting and preserving the Quran (sura)s in their memory.
2) during the life of the first caliphate, it was collected from the various written parts. Each aya(verse) were required to have at least two witnesses who heard the aya directly from the mouth of the messenger. this copy was saved.
3) during the life of the third caliphate (around 15 years) after the death of the messenger (peace be upon him). the islamic state was highly expanded, muslims were in control of egypt,syria, iraq, yemen and persia, all ruled by the caliph. the areas was so wide such that weeks of time were required from a place to another place inside the islamic state. many foreigners converted to islam while arabic wasn't their original language.
since the copying process of musshef (quran conveying book) was manual (someone should rewrite it again for each copy), a risk of copying errors arose, for all these reasons, the third caliph (Osman) decided to write reference messhaf, these reference copies were used as references for other people to copy from. the musshaf of the first caliph was acquired (since it was written by elaborate precise process) and copied many times, the procedure used in the time of first caliph were repeated. two witnesses were required for each aya.
it is to be noted here that the restriction that the two wittnesses should have (heard it directly from the mouth of the messenger) restricted the number of acceptable people for the witness role because many sahabas left al-madina to syria and iraq.
the preservation of quran didn't depend on written manuscripts only, it also depended on thousands of muslims that were (reciting/memorizing) the quran. the combination of both (written on paper) and (saved in memories of many people) gives this process a high confidentiality.
i hope this be clear because i have difficulty expressing my ides in english.