Is it possible for someone afflicted by jinn possession (mass), the evil eye (ayn), envy (hasad), or intense fear to be completely cured, and does the Qareen cause one to see the unseen, and what is your advice to someone who is weary of seeing the unseen revealed to them?

No one knows the unseen except Allah the Almighty. However, the jinn companion (qarin) may cast news into the hearts of some people, which it takes from the devils. Allah may also reveal some of the unseen to certain people through inspiration and divine discourse (tahdith), which scholars call "kashf" (unveiling). Kashf does not indicate the high status of its possessor, for what matters is rectitude and adherence to the Sharia, and success in obedience is the greatest divine favor (karamah). Unveilings can emanate from the righteous and the wicked, the believer and the disbeliever. Therefore, the person experiencing such states must be tested against the Quran and the Sunnah.