To what extent is it just for the worshipper to go to Hell and the disbeliever to go to Paradise in the mentioned story, and how can we reconcile the fate of one who grew up in a non-Muslim environment, adhering to good morals, with the fate of a Muslim who neglected them?

We have not been able to confirm the story mentioned in reliable hadiths, and what matters are the final deeds. Whoever concludes their life with a certain deed will be dealt with accordingly, as in the hadith: "Indeed, a servant may do the deeds of the people of Hell, yet he is from the people of Paradise; and he may do the deeds of the people of Paradise, yet he is from the people of Hell. Deeds are judged by their conclusions."

As for the disbeliever, his deeds will not benefit him because they are nullified by polytheism, as Allah Almighty says: {And whoever disbelieves in faith—then his work has become nullified} [Quran 5:53]. Allah does not wrong people in anything, and the disbeliever is recompensed for his good deeds in this world.

Sins are committed by both believers and others, but the believer repents, and good deeds may expiate some sins. The believer fears sin and repents, unlike the hypocrite. The believer will not abide eternally in Hell due to his monotheism, and disbelief is not forgiven.