What is the difference between an unalterable decree (al-qaḍāʾ al-maḥtūm) and a contingent decree (al-qaḍāʾ al-muʿallaq), and do the details mentioned for each apply to them?

Destiny includes what is predetermined and unchangeable (in Allah's knowledge and the Mother of the Book), and what is suspended (in the records of the angels), which involves effacement and affirmation.

Thus, happiness, wretchedness, lifespans, and provisions are fixed and do not change from Allah's prior knowledge of them. The proof for this is Allah's saying: "And Allah will never delay a soul when its appointed time has come," and Ibn Mas'ud's hadith about the writing of provision, lifespan, happiness, and wretchedness in the mother's womb, and Umm Habibah's hadith: "You have asked Allah for fixed lifespans and apportioned provisions, nothing of which will be delayed."

As for what is subject to effacement and affirmation, it is as in Allah's saying: "Allah effaces what He wills and affirms, and with Him is the Mother of the Book," and the hadith: "Whoever wishes that his provision be expanded and his lifespan be extended, let him maintain ties of kinship," and the hadith: "Nothing repels destiny except supplication." The increase in lifespan here is either through blessing in it and being granted success in obedience, or it is in relation to what appears to the angels in the Preserved Tablet, because Allah's knowledge does not change.

Supplication does not repel destiny because it is one of the strongest means of achieving what is destined. What is destined is decreed with its causes, and supplication is one of these causes.