No, the Crucifixion is not an event in time, or at least not solely so, whatever those who like to insist on the “scandal of particularity” may think. On the contrary, “the Lamb [i.e., Christ] was “slain from the foundation of the world” (Rev. 13:8). As I always tell my students, if we can properly use the adjective “eternal” of the Trinitarian Persons, then we must use the adverb “eternally” with respect to Their actions. Whatever the Father, Son, and Holy Spirit may “do”, They are “always doing” it, if we’re to continue speaking (as of course we must) in temporal language. It’s therefore false to say that God “made” the world, unless we add at once that He “is making” and “shall continue to make” it, instant by instant.
So also for all the “events” in the life of Christ: the Nativity, Baptism, Transfiguration, Crucifixion, Resurrection, and Ascension are all “ongoing” realities. Indeed the Second Coming has “already occurred”, as is affirmed in the Anaphora of the Liturgy of Saint John Chrysostom: “Thou it was who didst bring us from non-existence into being, and when we had fallen away didst raise us up again, and didst not cease to do all things until Thou hadst brought us back to heaven, and hadst endowed us with Thy Kingdom which is to come.” One cannot but think of a similar juxtaposition of tenses in Christ’s words in the Gospel, “Before Abraham was, I AM” (John 8:58).
I would hesitate, however, drawing quite the same conclusion you have—though I certainly agree your formulation is a tempting way of reconciling Christian conviction with the apparent Koranic denial of the Crucifixion (4:157). You wrote that “if Jesus was crucified before time, then the earthly event was simply a temporal manifestation of that pre-temporal reality, meaning that they [the Jews] certainly did not kill or crucify Him because it had already happened”. My sense is that your words “simply” and “already happened” are not sufficient for coming to grips with Christian teaching on this paradoxical point, namely, that what “happened/happens/will happen” on Golgotha is the eternal Self-sacrifice of the Lamb in Heaven.
As I was just explaining to my Theology class yesterday, the eternal isn’t really “pre-temporal”; supra-temporal perhaps, but nonetheless not something in relation to which a historical event could be thought to come later.