It is precisely because it IS real that it is dangerous and warned against.
Your conclusion is intellectually dishonest.
It is clear to see in the example you have given, that the teaching is the doctrine of the Witch of Endor, not of Adonai (God of Israel), an that Saul has subjected his belief to the spirit operating through her.
Thus, I rather say that it is because it is a teaching of demons that it is dangerous and warned against.
Unless you produce other scriptural support to represent what you say is Jewish teaching, I can only see that you have believed what the witch was teaching.
But all that aside, the Bible affirms that this really was the prophet Samuel who came *up* from Sheol. If even nearly spotless prophets went to Sheol then clearly everyone did.
I do not see evidence of that in what you have presented, while considering that king Saul had subjected his mind to a demonic spirit.
Classic Jewish cosmology. Keep in mind that all but one of the authors of the Bible were Jewish and all the early apostles and followers were Jews.
It has never slipped my mind ("First of all, they were entrusted with the oracles of God.").
"not all those who are descended from Israel are Israel, nor are they all children because they are Abraham’s seed; rather, “Your seed shall be called through Isaac.” That is, it is not the children of the flesh who are children of God; rather, the children of the promise are counted as seed.
"For one is not a Jew who is one outwardly, nor is circumcision something visible in the flesh. Rather, the Jew is one inwardly, and circumcision is of the heart—in Spirit not in letter. His praise is not from men, but from God."