Bauckham attempting too grand a theory

I’ve managed to read a few more pages of Bauckham’s Jesus and the Eyewitnesses and at times I get the sense Bauckham is attempting to address too many issues, without managing them sufficiently. For instance Bauckham follows Dunn in asserting that oral transmission should not be understood using a literary model:

We should think of each performance of an oral tradition as differing from others, but not in such a way that each builds on the other. With oral tradition there is no linear development, layer on layer. [p248]

Dunn does a good job of showing, through study of a range of examples from the Gospels, that parallel texts in the Gospels are best understood as varying performances of tradition... [p257]

As we have seen, against the form-critical conception of oral tradition operating like successive editions of a literary text, Dunn insists that each performance of a tradition is a performance of the tradition as such, not a further development of the last such performance. There are no layers of tradition. . . [p259]

But it seems difficult to maintain the supposition that there are no layers when it comes to oral transmission. How could the preceding performance cease to exist or cease to have any effect on the current performance? It seems a bit circulatory (and overconfident) to use the written Gospels as evidence for this (as Dunn does) against a written model.

It would be difficult to know when differences in the various Gospel accounts are non-editorial and free from any layers. Are the aphorisms and parables that Bauckham and Dunn have in mind to be taken as written versions of various oral performances independent from all other performances (oral and written) in all three synoptic Gospels? Some traditions (particularly in Mk) may have been written to be performed orally, but can we assume the same thing for Lk? Why see Lk or Mt as dependent on Mk for any of their traditions, if they each already knew them independently from Mk? This would seem also to do away with using a literary model for aphorisms and parables found in Mk and re-used in Mt and Lk. Is Bauckham aware of the ramifications of following Dunn?

At the same time it seems Bauckham also wants to warn us not to distinguish too sharply between written and oral transmission, doubting the opinion that "before the Gospels the Jesus tradition was purely oral and made no use of writing" [p251-2] and when it comes to memorization, oral traditions are suddenly to be understood on an analogy to the written model:

Although Dominic Crossan has argued that a sharp distinction should be made between these two [memorizing material from books or oral material], such a distinction is inappropriate in a society that, though predominantly oral, did make use of written texts. [p280]