So are you claiming Paul was just claiming not to invent the atonement despite inventing it? You say the idea cannot be traced any further back but this idea is throughout the old testament, both explicitly and in types and shadows. The copious references in the New Testament shows this idea was there throughout.
What Greek mystery religion predating Christ had the idea of a universal sacrifice for salvation? Curious to know.
Assuming the gospels were written half a century after Christ's death (there is a scholarly case for an earlier dating but of course scholars only seem to matter when they make points that you agree with so I concede this only for the sake of argument), they were written by EYE WITNESSES, so they can definitely be relied on to tell us what Jesus said, they were reporting events as they witnessed them not writing a biography of someone's life. I agree with you in the sense His sayings might not have been recorded word for word but rather to convey his idea but even if I grant you that it still shows the idea Jesus conveyed was that He is the atonement and sacrifice for the sins of mankind.
Isaiah 53 talking about Israel? Well Isaiah in the new Testament (including 53 which I cited) is quoted to refer to Jesus repeatedly. Secondly from a cursory reading of the verse, it is clear a man is being referred to not a nation. So what do you say about Psalms 22? Israel also?
It is interesting you critique interpretations which have the effect of making scripture say whatever you want them to say but yet you make Isaiah about Israel not Christ. I am also sure you would not agree with Old Testament scripture showing you the atoning work of the Messiah because it does not say what you want it to say.
In summary, the idea predates Paul, the idea of a universal sacrifice for the sin of mankind is unique to Christianity, the gospels can be used to justify what Christ said and did. Isaiah 53 is about Christ and makes no sense when read to refer to Israel.