1) According to Middrash Rabbah: Evil is sometimes allowed by G-d in order good to be known and manifested. G-d appointed a specific angel, Satan to perfrom this function and it is a function of Satan to challenge good. Jews do not believe in Devil. This is invention of Christianity. Satan or Sammael is someone who does not oppose G-d, but someone who is hired by Him to collect evidence on anyone misdeed.
6) The war between two seeds was between descendant of Adam and Samael, Serpent, who seduced Eve and concieve a child with her. There is no conflict between seed of Jesus and Satan as defined by NT. IT is again a conflict between two descandants who had the same mother. Depending on the source, the fall of humans in the Eden was caused by sexual immorality, not by disobedience eating forbidden fruit. There is no future conflict between two seeds, as Jewish Messiah will bring a peace to humanity.
7) No, I have not said that. Angles do not have inclination to do evil, they are good by the nature and presence of G-d. Angels who materialized on Earth and committed sin did it as a humans because they got free will (or freedom of choice). Angels who destroyed Sodom were punished on Earth because they bragged carrying the punishment on their behalf, not on behafl of G-d. Genesis 28:11-12 mentions a ladder between Heaven and Earth which is used by angels back and forth.