Does the lack of availability of the written sources of the past now (they were present back in the time then and some manuscripts are available today as well) negate the strong oral traditions present at the time ? Qur'an was transmitted orally and preserved by memorization primarily, along with copying them in writing by the scribes.
We know from the hadith (actions and sayings of the Prophet and his companions) and the Sira (history) literature regarding how the Qur'an was compiled, when it was done, who commissioned it and the companions involved in the process. Unless there is an alternative history (not speculations based on absence of evidence or bogus apologetic claims) challenging this, I don't find any reason to doubt them. The little available historical data only conforms to the scholarly tradition.
Take for example the recent carbon dating of the Birmingham manuscripts which dated them between 568 and 645 AD with 95.4% confidence, the latest date falls just 13 years after death of Muhammad(saw) [571 AD - 632 AD]
My comment was in relation to judging the prophets based on the "divine laws". Even if you don't consider them as divine, you could still consider them as laws applicable of the time they lived in.
most of it isn't today or maybe you think it is