How can one reconcile the fact that the torment of the disbeliever in the Hereafter will not be lightened—as you mentioned—with the lightening of the torment for Abu Lahab and Abu Talib?

The disbeliever will not benefit from his deeds in the Afterlife, due to the absence of the two conditions of sincerity and adherence (to the Prophet's way). This is affirmed by numerous Quranic verses, such as His saying, the Exalted: (And We shall turn to whatever deeds they did, and We shall make them as scattered dust) and His saying: (Their deeds are like ashes which the wind blows forcefully). The Prophetic traditions have stated that the disbeliever is recompensed for his good deeds in this world, and he will have no share of them in the Hereafter. As for what has been reported regarding Abu Talib concerning the alleviation of his torment, it is specific to him and due to the intercession of the Prophet, peace and blessings be upon him.