Are the strange ability to read minds and predict events, the feeling of detachment from the body, hearing inner voices, seeing terrifying nightmares, and wavering between faith and disbelief considered a type of demonic possession, and what is the legitimate Shar'i remedy for that?

Prayer is the pillar of faith and the distinguishing factor between disbelief and belief. Whoever abandons it has disbelieved, therefore, one must hasten to repentance. The knowledge of the unseen is something Allah has exclusively reserved for Himself, and none knows it but Him, except for a messenger whom He has chosen. What you see of dreams, the feeling of crosses, and knowing what goes on in the minds of others, is from the tampering of Satan due to your abandonment of prayer and commission of sins. Therefore, hasten to repentance, remembrance of Allah (dhikr), recitation of the Quran, accompanying righteous people, and keeping away from evil companions.