The Romans did not do things at the instigation of the Jewish Authorities. The Temple Priests were hand-picked lap dogs of the Romans and were generally despised as collaborators by the populace. If Roman soldiers were there, then the arrest was done under the authority- and the impetus- of the Romans. The Romans did not run errands for the priests and would not have cared in any case for what they would have perceived as an internecine religious squabble among the Jews. If the Romans were involved then something more than religion was in play, but more importantly, it meant that the Romans were the ones who wanted Jesus arrested. Whatever Temple authorities were there would have been there at the behest of the Romans, not the other way around. More on this shortly.
Exactly. John was not eager to shift or share any Jewish culpability if hecould avoid it, so the only sensible explanation for John putting the Romans in Gethsemane was because he believed it was true.
Now here’s my hypothesis:
It is now somewhat agreed that John probably had some knowledge of synoptic traditions even though he is not literarily dependent on them. There is also some argument that John may contain some authentic pericopes which are independent of the synoptics. In other words, some people think that John may contain elements of oral traditions which are not found in the synoptics but which are authentic, nonetheless. That’s not to say that John is anymore historical as a whole, only that it could have a few authend strands here and there.So here’s my guess. John did not add Romans to his story, he added Mark’s priests. The arrest and trial before the Sanhedrin is probably a complete fiction (it is rife with factual and procedural error), and was composed as an apolgetic device for Pilate as well as a polemic against Jewish authority.
I think there was probably some shred of oral tradition or general knowledge that the Jesus was arrested by Roman soldiers and brought to Pilate. John simply incorporated the synoptic tradition into an arrest story that originally didn’t contain it. This would make sense because it would show John shoehorning Temple culpability into the arrest and trial rather than inexplicably diminishing the role of the Temple by inserting Roman authoriies who had no literary reason to be there. It blunts John’s rant and serves no purpose in the story to throw a cohort of Romans into the arrest. I think John layering the synoptic elements on top of a pre-existing oral tradition makes more sense.