Is Barabbas just an allegory?
There's a simple reason the Barabbas scene cannot be an allegory: If the writer-creator's intent here was to paint an allegory, he would have done a much better job of it.
. . . as Richard Carrier points out, the sacrificing of one prisoner and the releasing of another is a human version of the 
Scapegoat ritual, where . . .
No, this is one more case of Carrier garbage in, Carrier garbage out. There is no analogy between the Barabbas story and the scapegoat ritual.
. . . where one of a pair of goats is slaughtered and the other is sent out into the wilderness with the community's sins transferred to it. JC was the slaughtered one and Barabbas the released one.
This interpretation is worse than accepting the literal story as is, of Pilate releasing a prisoner because it was a custom, as the account says. If they wanted to put a scapegoat parable in here, they needed to have an INNOCENT prisoner released in trade for Jesus (who was also innocent). Because the scapegoat ritual is about 2 innocent goats, not about one being guilty and the other innocent, like the case of Barabbas and Christ being exchanged.
The Gospel story clearly tells us about an innocent person being wronged -- betrayed, wrongly condemned -- in comparison to another who is guilty and yet being set free. I.e.,
two injustices happening together -- an innocent one condemned, and a guilty one going unpunished. There's no way such a scenario can be an allegory for the scapegoat ritual, which is about TWO INNOCENT individuals being made part of a ritual.
It's obvious that this Gospel scene cannot possibly have been invented/created by the author/editor as a metaphor for some religious symbol (at least not the scapegoat symbol) because in such a case the creator of the metaphor will pick out details which illustrate the metaphor rather than details which contradict it.
The account of this scene at the trial probably is tainted, because it's hard to believe there was such a
custom to release a known murderer. So we're probably not getting the whole story, and we're left to speculate.
The best explanation is this:
Jesus Barabbas (he was another Jesus) was an anti-Roman insurrectionist who committed murder in the recent riot. Jesus Christ was blamed for starting the riot, and yet if this were true he probably would have been arrested right then, just as Barabbas was. The truth more likely is that it really was Barabbas who started the riot, killed someone, and was arrested, as also some other rioters were arrested.
The deal -- Jesus for Barabbas: Judas Iscariot was an anti-Roman militant in sympathy with Barabbas, and both of them had hoped Jesus Christ would use his power to initiate an anti-Roman uprising. But this was not the intent of Jesus, whose plan was not an insurrection. So Judas became disillusioned with Jesus and turned against him, deciding to make a deal with the Priests in Jerusalem to have Jesus arrested, on the condition that Barabbas would be released. The Priests were more afraid of Jesus as a threat -- as a rallying point for insurrectionists attracted to him -- than they were afraid of Barabbas, who was a common militant posing little threat to them or to the Romans.
So this exchange of Jesus Christ for Jesus Barabbas was agreed to, and this resulted in a scene where Pilate formally releases Barabbas in exchange for Jesus. Many in the crowd who cried "Crucify him! Crucify him!" were anti-Roman militants in sympathy with Barabbas, and also angry at Jesus for failing to lead the expected insurrection. This fits the scene described in the Gospels, though the explanation presented there is partly erroneous, or confused. But a deal arranged by Barabbas and the Priests fits mostly the scene described (in the Gospel accounts).
Some of the scene, as told in the accounts, is still unclear. But obviously this trade deal -- between Judas and the Priests -- makes much more sense than a nutty metaphor explanation trying to make this a parable for the scapegoat ritual.
BUT, would Pilate agree to release Barabbas? because of a deal between the Priests and Judas? Maybe Barabbas was popular among the militant anti-Roman dissidents, and many of them were at this "trial" and expressing anger. Though Pilate may have been reluctant to honor such a deal to release a known criminal and murderer, still the crowd of dissident militants demanded that the trade be done, that Barabbas be released. And Pilate yielded to this, rather than risk unnecessary violence. He too probably was persuaded that Jesus was more of a threat than Barabbas, and so figured that the trade was reasonable.
Whether the above is the explanation, all four accounts say the following scenario took place -- a criminal released and Jesus who was innocent being condemned instead. When all the sources agree, there must be some truth to it, and it cannot be a total fabrication. Or, you are rejecting the evidence if you assume it's a total fabrication. It's better to acknowledge that the general scene is accurate (attested by all the sources), while there might be inaccuracy in the details of the scene. Confusion of the details is easy to explain as normal in a conflict scene involving anger and emotion and hate between rival factions, as happened here.
Treating the Gospels as documentaries leads to absurdity after absurdity.
No one treats them as documentaries.
They are accounts describing someone's version of what happened (4 different versions), also containing interpretation and fact mixed with fiction --- like most of the ancient writings, including the historical writings we rely on for the history. It's reasonable to believe the reported events where the sources agree, but to doubt them where there are discrepancies.
(That there was a "custom" to release a prisoner at Passover is mentioned in 3 of the Gospels, but not in Luke.)
What is agreed by all 4 sources is that Barabbas was guilty and was set free, while Jesus was innocent but was condemned; and somehow the two are together in the "trial" scene, with Pilate making some offer and the crowd demanding Jesus to be condemned and Barabbas to be released. And Jesus was betrayed to the Priests by Judas, whose motive isn't clear.
This cannot all be allegory. Even if there is some allegory in the Gospel accounts, this isn't it, because all 4 accounts say the above, and all 4 writers/editors would not create the same allegory, and especially an allegory which makes no sense or has a flawed correspondence between the objects in the allegory, meaning all 4 of them, separate from each other, made the same mistake, committing the same flaw or misapplication of the scapegoat symbol.
So with all 4 sources in agreement on these main points, we should assume there's some truth here in the scenario presented, of the swap, Jesus condemned in return for Barabbas being released. This has the earmarks of being based on a real event, not allegory, with some detail or explanation of the event left unclear.
