Why did the Qur’an not mention the torment of the grave and its constriction, while it mentioned the torment of the Fire and the reckoning? And does the absence of their mention negate their existence, especially given the possibility of some hadiths being fabricated?

It is not a condition for all beliefs and rulings to be detailed in the Qur’an for them to be established, for the Prophetic Sunnah is a revealed inspiration and an elaboration of the Book. Imam Al-Shafi’i stated that the Sunnah is of the same status as the Qur’an. The torment and bliss of the grave are established by evidence from the Qur’an and authentic Sunnah, and whoever denies them is astray and deviant, and disbelieves what the evidence indicates. Allah Almighty has commanded belief in the unseen matters He has informed about in His Book and through the tongue of His Messenger. The statement that the soul leaves the body, thus there is no life or pain in it, is refuted by the Hadith of Al-Baraa, which affirmed the return of the soul to the body in the grave.