The devil is the leader of the demons....he commands them. They are his henchmen.
The devil is the cause of the demons. For example, 1Jn_3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
Now, James say that LUST gives birth to sin and sin death. So, when John says "he that comitts sin is of the devil", he is say that the sinner is serving his own LUST. And the devil who sinned from the beginning was the LUST of the flesh of Adam and Eve.
Adam's sin has brought death to everyone. That means that all forms of physical and mental illnesses we encounter on our way to death are charged against the devil. Which is to say they are charged against the LUST or SIN in the flesh.
Who did the witch of Endor summon? Who are the ones who communicate through spirit mediums and give them information?...often surprisingly accurate information...? It is a deception designed to promote the devil's first lie.
If you read the text, the woman never summoned anyone. Samuel appeared before she could do anything. And in fact, she was frightened when she saw gods ascending from the earth. Mediums are frauds.
Why are they called "spiritistic".......who are these spirits that Jesus and his apostles expelled?
What Jesus and his apostles were doing was healing people of all manner of both physical and mental illnesses. The devil is a personification of the lust of the flesh which has brought those illnesses upon man when Adam sinned and brought death into the world.
When an evil spirit was expelled the person was cured of the illness they had. And just as the devil is personified so too are his works that have caused illnesses unto death.
It is your belief that the devil is not a real entity.....but that is not what the Bible teaches because the demons are satan's army of rebel spirits. They can inhabit humans and even swine apparently. (Matthew 8:28-32) Do swine suffer from 'lusts of the flesh'?
All spiritistic practices are communication with spirits. That is why its call "spirit-ism".
When Jesus cured the man back to his right mind the illnesses he had were transferred to the swine.
The Devil has his angels too. In his judgment of the "sheep and the goats" Matthew 25: 41 clearly says.....
"Then he [Jesus] will say to those on his left: ‘Go away from me, you who have been cursed, into the everlasting fire prepared for the Devil and his angels."
The devil and his angels that Jesus speaks of is Paul's man of lawlessness. he is called the red dragon in Revelation. He is a man who will embody all the wickedness of the flesh. And his angels are those who will follow him. When people who are in covenant with God follow after their own evil lusts they will be cast into the lake of fire as the man of lawlessness and his angels are.
Jude 6 tells us about "the angels who forsook their own proper dwelling place" to materialize human bodies and indulge in the pleasures of the flesh in Noah's day.
"And the angels who did not keep their original position but forsook their own proper dwelling place, he has reserved with eternal bonds in dense darkness for the judgment of the great day."
The sons of God who went whoring after the daughters of men were human beings who were following God's way and who had taken upon His name (sons of God) but then went after the daughters of men who were out of God's way.
To the Pharisees Jesus said...
"You are from your father the Devil, and you wish to do the desires of your father. That one was a murderer when he began, and he did not stand fast in the truth, because truth is not in him. When he speaks the lie, he speaks according to his own disposition, because he is a liar and the father of the lie."
Jesus says that they were following after the LUSTS of their father, the Devil. The Scripture says that Cain was of the evil one and he murdered his brother. Cian was of the evil one because he allowed his flesh to become enraged in jealousy enough to kill Abel.
So I don't know how Jesus can speak about satan as a real entity and be tempted by him (since Jesus was sinless)....and talk about his angels (satan's angels are the demons) and expel them from humans and allow them to invade a herd of swine, if they don't exist except as concepts of evil in humankind.
Your interpretation does not line up with the rest of scripture.
May I suggest you start by studing how sin and lust are personified in Scripture.