Regarding "the lack of references to him in our earliest Roman Christian literature," it is true that Paul's letter to the Romans does not mention Peter. But the Letter of the Romans to the Corinthians (1 Clement), written near the end of the first century CE, refers to the martyrdoms of Peter and Paul in chapters 5-6. It doesn't specify the location of the death of either.
There is a tradition that Paul too was executed in Rome, but like the one about Peter, it is late.
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