How can it be just for an unbeliever to abide eternally in Hellfire for disobeying Allah for a short period, and is it decreed for those with disabilities who die as unbelievers to suffer in this world and the Hereafter?

The eternal damnation of disbelievers in Hell is due to the fact that, had they remained in the world, they would have persisted in disbelief, as evidenced by the Almighty's saying: "And if they were returned, they would return to that which they were forbidden," and because people will be resurrected according to their intentions. As for those with disabilities, if the حجت (proof/argument) was established against them and they did not believe, they deserve eternal damnation in Hell, with varying degrees of torment according to their deeds and their tribulations in the world.