In innā sawfa tudrikunā l-manāyā, inna provides emphasis, nā is its subject, and the verbal clause forms its predicate. Al-manāyā is the subject of tudriku; the attached nā is the object.
Muqaddaratan lanā wa-muqaddarīnā sets up a parallel relation between allotted deaths and humans subject to the same decree. The rhetoric doubles the sense of inevitability.