The nominative word is the object of the sentence.
مُسۡتَقِرٍّ
mustaqirrin
“and every established matter (will occur)”
In the genitive, this is most likely an adjective for amr. The phrase beginning with kullu is either conjoined to "the Hour" in the first ayah, or it is the subject of an implied predicate.
Explication
These readings amount to the same meaning, though the latter can be interpreted in several ways [al-Alusi].
He has revealed to you ˹O Prophet˺ the Book in truth, confirming what came before it, as He revealed the Torah and the Gospel
Explication
These readings amount to the same meaning, though the latter can be interpreted in several ways [al-Alusi].