Who Would Invent a Crucified Messiah?

Liberal scholars like Bart Ehrman and James McGrath argue that one of the ways we can know that Jesus was a historical person is that first century Jews had no expectations that the messiah would suffer and die.  At the time, all Jews believed that the messiah would be a conquering hero. Therefore, the only explanation for this belief arising is that someone who was believed to be the messiah by his followers actually suffered and died.

Christians claim that the prophet Isaiah predicted that the messiah would suffer for the sins of his people:

He was despised and rejected by mankind, a man of suffering, and familiar with pain. Like one from whom people hide their faces he was despised, and we held him in low esteem. Surely he took up our pain and bore our suffering, yet we considered him punished by God, stricken by him, and afflicted.  But he was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was on him, and by his wounds we are healed.

Isaiah 53 3:5.  However, Ehrman says that first century Jews didn't understand this as a messianic prophecy.  Rather, the prophet is describing the past suffering of Israel during the Babylonian exile.

What I find interesting is that Christian apologists use the same logic to argue that Jesus was really resurrected from the dead.  Since no first century Jews expected the messiah to suffer and die, the only explanation for this belief arising is that Jesus of Nazareth proved he was the messiah by rising from the dead.

Personally, I don't think we really know enough about how an idea like this might have been invented to say what must have happened to cause it.  Ehrman writes "Who would make up the idea of a crucified messiah?  No Jew that we know of."  So what?  Prior to Joseph Smith, did we know of any Christians who would make up the idea of the Golden Plates and the Angel Moroni?  Does that give us any reason to think that there is anything historical about Smith's stories.

I think it entirely plausible that the idea arose of a crucified messiah because the follower of an executed messianic claimant interpreted Isaiah as a prophecy in order to cling to his belief in the man he had followed.  However, I don't see how that makes it highly probable and I don't see how that is the only way it could have happened.  Given the number of devout Jews who must of been searching their scriptures in order to understand why God had not sent a messiah to deliver His people from their tribulations, I think that any number of people might have stumbled on the idea that Isaiah 53 3:5 was a prophecy.

Moreover, even if we could establish that the execution of a real messianic claimant is the most likely circumstance under which a first century Jew would come up with the idea of a suffering messiah, does that mean that it must have been one of Jesus' followers who did so?  Potential messiahs were a dime a dozen if first century Palestine.  It's equally likely that it was a follower of John the Baptist who stumbled upon the idea in an effort to understand his death or the follower of one of the many other messianic claimants of the day.

I was recently chided by Dr. McGrath for making such suggestions:

[T]he existence of sources which say things that are radically different than the ones we have is itself a mere possibility, which cannot be excluded but neither should it be assumed to be probable. And so we should and do assess historical probabilities using the evidence we have, not the evidence that we could theoretically have.

I could not help but note that the sources we have say nothing about the idea of a crucified messiah arising from someone's attempt to maintain their belief in a messianic claimant in the face of his execution.  The sources we have say that the belief in a crucified messiah arose from Jesus of Nazareth literally rising from the dead and appearing physically to his followers.  The whole enterprise of trying to determine the actual events that might have led to the development of such stories necessarily involves a great deal of speculation and conjecture.