No doubt there is similarity of such sentiments to some of the Sermon on the Mount and in other places as well, but these words and the sequence isn't even close to being identical to any Gospel passage
First, you forgot the other passage I cited from 1 Clement. As for the rest, I will address it below.
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if they were, Oberon no doubt would have laid them out for us
I did:
My mistake. I meant 13.1-2:
"Let us especially remember the words of our Lord Jesus Christ which he spake when teaching gentleness and long-suffering, for he spake thus:
13:2 Show mercy, that ye may obtain mercy; forgive, that it may be forgiven unto you; as ye do, so shall it be done unto you; as ye give, so shall it be given unto you; as ye judge, so shall ye be judged; as ye are kindly affectioned, so shall kindness be showed unto you; with whatsover measure ye measure, with the same shall it be measured unto you."
Scholars such as R. M. Grant (The Apostolic Fathers, vol.1: An Introduction, p.40) admits that Clement’s source is probably oral.
Some scholar's do indeed argue this.
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
On oral tradition in 1 Clement, see point #3
This is absolutely false. He is not in anyway the leading authority in this subject. Koester's specialty has always been gnosticism and extra-canonical texts. This is not to say he is in anyway ignorant of 1 Clement and other early christian literatue; he is well acquainted with them and scholarship about them. But he is in no way the "leading authority."
Now for refutations:
First, 1 Clement was not the only example I gave. The earliest references to Mark are Matthew and Luke, two authors from different communities who independently had access to mark, less than 2 decades after it was written. Others I already mentioned.
As for 1 Clement, there are a few possible explanations:
1. As many argue (probably a consensus, but there are many who disagree), 1 Clement shows awareness of the gospels. His quotes are sometimes almost exact, but there is a lot that isn't. However, Matthew and Luke knew Mark, but changed a great deal of his gospel. It was common in christian lliterature, even in the centuries where the gospels and christianity dominated, for christian authors to quote the gospels inexactly, or combine them, or alter them. This happened for a few reasons: First, they often quoted from memory, and so were inexact. Second, rather than just quote a passage found in Matthew and Luke, or in all they synoptics, they would combine them into a similar but altered version (due to the combining). Third, often enough the exact quote did not fit will into the text, and was altered so that it could.
2. 1 Clement was not aware of any of our gospels, but rather quoted sayings of Jesus from other texts based on the gospels. But this would mean that THOSE texts, prior to 1 Clement (and thefore 1st century) knew the gospels.
3. 1 Clement relied on oral tradition (as you suggested). If this is the case though, it is an indicator of GREATER reliability of the gospels, because it show a controlled oral tradition in the christian communities. If 1 Clement used an oral source independent of the gospels, but his quotes were often very close to the gospels, this means that the oral tradition in the Jesus sect was controlled enough that a guy living in the end of the first century knew teachings of Jesus through oral transmission which are also found in earlier texts. In other words, if 1 Clement has access to a reliable oral tradition concerning Jesus in c. 95-6 CE, how much more accurate would the oral tradition be for Mark, writing c. 70 CE?
Any way you look at it, 1 Clement supports the historical Jesus. If he was aware of the gospels as many or most scholars believe, than he adds to my point about the widespread usage of the gospels almost as soon as the were written. The same is true if he relied on a text which used the gospels. If, however, he relied on oral tradition, than Jesus' teachings could be reliably transmitted 60 years after Jesus' mission.