It’s not plausible that J of A would have made a deal with Pilate given the nature of the charges (claiming to be the “King of the Jews”). That was a challenged not only to the authority of Pilate, but to the authority of Caesar himself.
The four Gospels are not independent records. Matthew and Luke both copy liberally (and verbaitim) from Mark (Matthew, in particular, contains about 80% of the Mark copied verbatim), so the presence of J of A in those other synpotics means nothing.
The presence in John is a little more unusual, but it’s not unique to find a few stray references to Mark found in John. John was written 30 years later than Mark, which is ample time for J of A to have entered oral tradition. Lets also not forget the smoking gun that “Arimathea” was not a real place, but a clear literay construction.
Yes, it’s plausible to identify Jesus as a peasant. He is explicitly identified as belonging to the tekton class, which was a class of sub-peasant, subsistence day laborors. The fact that the Gospels say he was called “rabbi” does not mean he was any kind of ordained clergy in the modern sense. That significance of the word arose after the destruction of he temple in 70 CE, when the primary focus of Jewish worship and practice shifted from the Temple to study of the Torah. That word in Jesus’ day just meant “teacher” and did not imply anything about economic status.