You ask about the had?th that “marriage is half the religion”. As a Muslim, you obviously know much better than I how your tradition interprets this axiom.
But since you’ve asked, I’ll risk a suggestion or two. If religion consists in our return to God, and if this journey home requires self-effacement and a death to our ego, what could be a better foundation than the crucible of married life, where each partner is offered repeated opportunities to give way to another that is intimately united to himself—with whom he is indeed “one flesh”? We are of course truly one Self with all men—which is why we’re told to love the neighbor “as the self”, and not merely “as if he were the self”—but in our wives and husbands we have a chance to come to terms with this fact in a context where we’re already predisposed to wish the other well.
Of course, the question will then arise as to why this is only “half”, and not the whole, of the journey. What more remains? Part of the answer, surely, lies in the fact that love of neighbor, even the neighbor to whom one is married, gets us nowhere without love of God. Another answer may be found in Krishna’s admonition in the Gita—that we must learn to detach ourselves from the fruits of our actions, even (or perhaps especially) those that come from the action of loving our spouse, sacrificing those delicious fruits in the fire of the knowledge that we are not the Actor.
Regarding your idea that marriage creates a “third thing” in addition to the two spouses—what you call a “union of essentials”—this may be true in some cases. But I suspect this mystical tertium is going to be rather rare—something we may certainly seek in this life, but without its being fully realized while we’re still in the body. You sound disappointed that even the closest of terrestrial relationships must always retain an element of “distance” and “mystery”. I agree that this can be “unsettling”, but isn’t that in fact just as it should be? For in confronting the unknown in another person we’re confronting, indirectly, the Unknown itself, in whose image the other person is made.
A proving ground for our growing knowledge of God: this, it seems to me, is a sufficient ideal for any marriage to aim toward. And then, if Heaven in its wisdom and generosity decides to make of it something more—to provide a foretaste of the supreme bliss we’ll partake of when all are in each and each in all—then of course Deo gratias.