Of course it's relevant... If the letters of Paul are our first point of contact to the "christ" then how the next stories develop are just as important.
Do you realise that the stories of the Christ follow the letters of Paul by at least a generation?
How long is a generation, or two?
I presume it is how long you want it to be Thrud.
I noticed on TV the other day, Paul McCartney being interviewed by the great and the good on his life and times. How long since the Beatles? Over 50 years. My father died recently and would relate stories of his teens during WW2 and that was 70 years before.
All the epistles are are extant ancient letters. If it is literary sources we are thinking about I can certainly, given the amount of paper work I dispose of per year in the course of my work believe that there was more rather than less.
But let's look at this from your point of view. You think Jesus just preached the Golden rule....which somehow in his hands......is a bad thing...but the words as I have loved you are a later addition.
What do you think the motive was for adding 'as I have loved you' to the golden rule?