How can marriage be both a preordained decree and a suspended decree at the same time, considering that a preordained decree is not altered by supplication?

There is nothing further to add to what we explained there in the referenced fatwa. The suspended decree (al-qada' al-mu'allaq) is what is in the hands of the angels, and the decisive decree (al-qada' al-mubram) is what is with Allah Almighty. Allah has predetermined the measures of all things fifty thousand years before creating the heavens and the earth, and that is written with Him in Umm al-Kitab (the Mother of the Book), and this does not change. As for what is transcribed for the angels, that is where the suspended decree occurs; and the angels do not know which of the two matters will happen until it occurs. As for Allah, Glorified be He, He knows what will be. There is no contradiction between the decisive decree and the suspended decree. Matters of divine destiny (al-qadar) should not be delved into, and the Prophet, peace and blessings be upon him, said: "When divine destiny is mentioned, refrain." Divine destiny is the secret of Allah; no close angel nor sent prophet has been informed of it. It suffices for a person to believe in divine destiny, both its good and its evil.