Will those who commit major sins, as described in the Hadith: "They will not enter Paradise, nor will they perceive its fragrance," necessarily be punished, or is their matter up to Allah, for forgiveness or punishment?

The perpetrator of a major sin is exposed to Allah's wrath and punishment. Allah may forgive him or punish him according to the extent of his sin, except for the disbeliever who is deserving of eternal hellfire. Severe warnings have been authentically reported in sound hadiths for those who display their adornments (تبرج). This does not imply their disbelief or their eternal damnation in Hell, like other sins, as long as they remain Muslim. They are subject to Allah's will; if He wills, He may forgive them, and if He wills, He may punish them. Eternity in Hell is exclusive to disbelievers. As for disobedient monotheists, they will be released from it through intercession or Allah's mercy. The warning (of punishment) is lifted from its subjects through repentance, or expiating good deeds, or expiating calamities.