(Grant appeals to Helmut Koester, who is generally regarded as the leading authority on the subject of the Fathers dependence on oral tradition rather than on written Gospels: see his Ancient Christian Gospels, p.14-20.)
Again, this statement is incorrect. Koester, as I said, is a well-known and excellent scholar. He is by no means, however, considered "the leading authority on the subject of the Fathers dependence on oral tradition." He is one several scholars who argues this, but more experts argue against him. It is a moot point, however, as
1. There are several other early early references to the gospels, Matthew and Luke using Mark prior to 1 Clement was written
2. If Koester's contention is correct, it means that the oral Jesus tradition did, in the first place exist (as you denied), and also that it was controlled enough to retain several sayings found in 1 Clement, 60+ years after Jesus died, which are found in the synoptics independently of clement. It also means we can add 1 Clement to our independent sources attesting to Jesus.
You say this without having ever done the research. I cited scholarship going back a hundred years, as well as a number of more recent studies which disagree.Clement's knowledge of any of the Gospels has never been satisfactorily demonstrated.
It became common practice to attribute all manner of sayings and teachings to a Jesus towards the end of the first century. Thomas is an example of a sayings Gospel that simply consists of a list of over a hundred teachings and sayings, all beginning with "Jesus said."
And Thomas is either dependent on the synoptics for much of his sayings, or is another independent (and some argue earlier, though probably more argue later) collection of Jesus' sayings. It is quite clear, however, that this is the same Jesus. Moreover, numerous sayings in Thomas have parallels elsewhere. Thomas is, if anything, another source for understanding Jesus. Koester, and his students, for example, use Thomas as an early and independent sources for Jesus' teachings.
Your methodology is not only flawed, it is the reverse of what is perhaps the most important tool of historical study of any subject: when you have multiple independent sources for a historical figure, you have MORE evidence, not less.
You argue that sayings were simply "attributed" to Jesus all over the place. And in part this is no doubt true. But they were all based on a single figure, identified as being the same in all the sources. Unlike figures such as Antiphon, who not only have a variety of vastly different references about and/or texts attributed to them (and in the case of Antiphon, are still considered by most to be the same person) our independent sources not only show extensive agreement, but they are all attributed to one Jesus Christ, and there is no indication anywhere that there were multiple such teachers/prophets who were eventually conflated.