How to “envisage the essential points of Christian doctrine” sub specie aeternitatis? It seems you’re well on your way to doing that already. You adduce Eckhart, but one needn’t look only to the works of an “alleged heretic”! The essential ideas—that the Logos has “always” been incarnate and that human nature as such, not the humanity of only one historical individual, has been assumed by this Logos—are implicit (when not explicit) in both Scripture and Tradition, including the classic Patristic formulations of Christology.

The Fathers were not metaphysicians, of course; this goes without saying, and there should no surprise in the fact that they presented the Truth “dynamically” and in reference to a “historical transformation”. But thinking of Christ’s work in these terms need in no way conflict with a metaphysical perspective. One temporal event can’t happen at two different times—I leave aside here any “quantum” objections!—but there’s nothing to prevent a temporal event from having a trans-temporal root. These are different orders altogether.

In any case, here’s how I myself attempt to sum up the metaphysical essentials of the Faith roughly half way through my lecture course on Christian Theology, as we begin transitioning from Christology to Soteriology:

We’ve learned that a transcendent and yet immanent Mystery, whose center is everywhere and circumference nowhere, is (even as we speak) emptying Itself into our world and ourselves, at once creatively and re-creatively, bringing us and all things into being from the No-Thing It is while at the same time redeeming and restoring those who, inevitably and yet reprehensibly, are falling away into a nothing It is not.

Now it’s up to those who are falling away to respond, and this they do in two ways: (1) through their trust in the fact that the world-restoring operations of the Mystery have already achieved their goal, and (2) through their recognition that, paradoxically, they are nonetheless responsible for achieving this very goal for themselves, which they can do by mirroring the operations of the Mystery, willingly emptying themselves into It even as It empties Itself into them, in order that finally they might become what It is.

As you’ve no doubt already intuited, the effort to “properly reconcile the temporal and the eternal in the Christian message” is not a scholastic exercise, but a matter of spiritual practice.