“Okay, so please explain to me how Iyov could have existed and not existed. Explain to me how Beis Din could have executed a BSUM and yet never executed one.”
This is my personal take on the subject. Whether or not Iyov existed is not so important. As with Midrash, it is the LESSON we learn from his story that is important. The idea that someone who has everything wonderful in his life but fails to worry about or daven for others less fortunate than himself, and that we need to be empathetic to others who are suffering, is one lesson. The idea that everything can be taken away in the blink of an eye, and therefore we are all equal in the eyes of Hashem, is also a lesson. The idea that nothing is in our control is another lesson. And finally, the lesson that no matter how bad things seem, they can always change. Just as the good does not last forever, neither does the bad, and the bad is a way of teaching us things that are meant for our benefit.
In the case of the BSUM, I would believe there might have been such a person, but even if not, we are being reminded what is supposed to happen in such a case. The description in graphic detail of a B”D execution, is enough of a deterrent for most people.