Since we are discussisng the development of traditions the opinion held by influential writers like Eusebius is significant. He doesn't even seem to suggest a controversy about the identities. IMO since his opinion did not prevail there must have been a powerful motivation to merge the characters. As the Arian controversy (reshuffling of names of brothers and apostles to disguise humaness) and political establishment of the Catholic Church (founded upon Peter) were in the air we might have already found the needed motivation.
Pete,
I found the reference of Eusebius you quoted earlier--about Cephas and Peter. All it seems to say is that one of the seventy disciples had the name Cephas, same as Peter. The only mention of Simon in the early history is of Simon Magnus, the sorcerer. Eusebius does mention Peter often.
So couldn't there have been a Cephas, a Cephas aka Simon aka Peter, and a Simon--three different men. I see no controversy there, nor do I see why they had to be merged into one figure.
It really doesn't matter which Cephas Paul refers to. If it was Peter, maybe Paul just had a preference for using that name for personal or cultural reasons. People of the time he was addressing would have known who he meant.