I think the more interesting and subtler point is that if these writings are inspired by God as is usually claimed why would there be any textual variants at all?  God can inspire the text but not maintain the purity of the tradition? 

The primary source should be the mass recurring oral group memorizing and passing the memorized materials over and over to others. 

The problem with oral traditions is that they’re even less reliable than written ones.  With textual variants you can at least suspect that one of the variants is original but if someone changes an oral story how will anyone who has never heard it before tell any difference?  And what if the person in charge of memorizing the story dies before he or she passes it on?  Whole traditions can be snuffed out instantly.

The only way for there to be a successful revelation from god is for him to give it to each individual personally.  A real god would have figured that out a long time ago.   

…the mass recurring oral memorization and transmission from one generation to another without any alteration…

The problem is that this is not how it worked.  

Certainly.  There has been a great deal of scholarly study applied to the differences between oral cultural transmission and literary cultural transmission.  Believe it or not there are contemporary cultures who survive by primarily oral transmission.  What the scholars find is that many of the qualities we ascribe to oral transmission were in fact developed as a result of literary transmission.  One of these qualities is the concern with memorization.  In oral cultures they didn’t care about this.  

In oral traditions the performer or storyteller shapes his story to fit the audience.  He is expected to improvise.  I heard of one fellow who told his story to two audiences.  One story lasted twenty minutes.  The other lasted forty minutes.  The storyteller said he told the same story to both groups!  He wasn’t interested in word for word repetition.  He was interested in sharing the ‘jist’ of the tale and fitting the details to the audience.

Memorization comes along after a culture has become literary.  There are folks who can recite the entire Koran from memory.  But the Koran was written down first and then and only then the concern becomes getting it correct word for word.

Oral transmission is more like Jazz, improvisation on a theme, rather than classical music where the intention is to reproduce the piece correctly note for note. Literary transmission is like classical music and has the concern for getting it “right”.

The idea that Jesus’ disciples memorized his sayings and had their followers do the same is not how it worked.   Jesus’ followers told stories about him.  They shaped them and reshaped them and molded them to their audience.   At some point the then current versions were written down.  At that point the transmission became literary andf people started obsessing over what the story really meant and what it actually said.