Although we are concentrating on the Third Act, Faust's appreciation of legend's most beautiful woman begins much earlier, perhaps as early as the Hexenkueche scene where he is thoroughly enraptured by a woman's image in a magical mirror. It drives him crazy, he says (2456), particularly since he has to stay at a certain distance to keep it in proper focus (2434); can you see Mephisto's mischievous smile at this bit of enforced "disinterested contemplation"? Woman is God's final art work, the true Crown of Creation, we learn from Schiller's Princess Eboli; Mephisto seems to say as much, and Faust like most men needs no convincing. We can't be sure that this is indeed Helen, we (and Faust) have yet to meet her. She remains nameless but, by any name, would be as sweet.

Act I finds Faust and Mephisto at the Imperial Court entertaining the Emperor and his entourage. We have encountered this function in the Faust Book and in Marlowe. Although the setting remains festive, even more so than in the earlier renditions, individual episodes, a veritable Bretton-Woods event in the introduction of paper money drawn on the nation's buried treasures, and the appearance of Homer's celebrated lovers Helen and Paris, have significance beyond their immediate entertainment value.
In the latter episode it is Faust, in contrast to the earlier treatments, rather than Mephisto, who is the principal actor. Mephisto will furnish a magical key which in turn will allow Faust to retrieve the magical tripod which will empower him to conjure up the two lovers. Faust will find the tripod in the Realm of the Mothers, an alternate reality without  the dimensions of space and time, a nothing nowhere, yet inhabited by The Mothers whom Goethe steadfastly refused to identify. All of creation swarms around them as images, shades, they cannot see Faust.
Of course we would like to discover who they are, but the Mothers are hardly the focus of what is to follow. What comes next is high adventure and a test of courage. Faust is willing to risk his life to explore a final frontier of sorts, "to boldly go where no man has gone before" (6399). And remember, not to get to know the Mothers, or to get Helen, but to experience a realm Mephisto calls "Nothingness." (*) Traditionally the descent (the rise?) into the underworld is required of one who would accomplish the extraordinary. Fear and terror will be encountered and must be conquered, they may even aid the experience (6271 ff.) There is a faint reference to the German fairy tale of the boy who set out to learn fright  (6272). Even Mephisto isn't sure whether Faust will make it back (6306).

(*) Again, please consult Henning Genz, nothingness. The Science of Empty Space. Helix Books. The original German title is Die Entdeckung des Nichts. Leere und Fuelle im Universum. Carl Hanser / Rowohlt.

The Magic Flute too lists the descent into the underworld as the hero's ultimate test. Please check the relevant chapters in Brigid Brophy's brilliant Mozart The Dramatist  and every time you read Tamino (and Pamina) substitute Faust. It will shed more light on the episode than trying to figure just who the Mothers are.
Faust does return with the tripod, touches it with his key, there is dense fog, then music that makes everything resonate, and from the mystery of organized sound and amorphous vapor Paris appears, then Helen. She's pretty, doesn't appeal  to me though, is Mephisto's verdict (6480). Faust's reaction is equally predictable. He is at once engulfed in passion, love, idolatry and madness (his words, 6500).
When the two phantoms reenact the abduction, Faust forcefully intervenes on Helen's behalf, there is an explosion that knocks him unconscious, and Mephisto is left with a mess. The scene ends in darkness and commotion.

We don't know what  his chances of recovery are, if any, until Homunculus allows us to witness a dream that envelopes the still unconscious Faust. Leda and the Swan. We know instantly that he'll fully recover. Not only is it an exquisite erotic dream, he moves into Helen's circle as it were, for she's the off-spring of the union in the grove.

I am tempted to believe that Goethe was familiar with Correggio's painting that graces the cover of your Oxford edition. Read the dream carefully (6903 ff.) and tell me what you think. The history of the painting reads like the story of Helen.
Painted around 1532 it moved from Mantua to Philip II in Madrid, then on to Prague and into the collection of Emperor Rudolf II. Swedish troops made off with it during the Thirty Years War, but Swedens's Queen Christina later took it with her when she moved to Rome. Philippe of Orleans bought it and brought it to Paris. Frederick the Great had it in Sanssouci, Napoleon took it back to Paris, by 1830 it was back in Berlin's Gemaeldegalerie, it surfaced in Washington in 1945, was returned to Berlin in 1949 and now hangs in the new Gemaeldegalerie, next to the Neue Nationalgalerie and opposite the Staatsbibliothek. Go see it and let it work its magic on you.
In a fit of rage (you explain it) Philippe of Orleans is said to have cut it to pieces; it was restored, with Helen's head painted anew, but her Leonardo-like expression certainly is not Correggio's.

At the beginning of Act III two distinctly different ages have merged, the Germanic and Slavic Age of Migration (4th cent BC through 6th AD) and prehistoric "Homeric" Greece. Moreover, the architecture of Faust's castle is medieval and Faust himself a medieval knight (or war lord) rather than a hero (or villain) from a feudal epic.
The Trojan War is over, Troy is destroyed, Helen is "liberated" and, she soon learns, to be sacrificed by her still enraged husband Menelaos, along with her Trojan servants. Like Faust at the beginning of Part II, Helen needs to forget all that has happened in the past ten years, the bloodshed has soiled every aspect of her life with Paris  (8870), who now appears merely as a kidnapper in her memory (8512). The chorus (8697) chants a lengthy account of the horrors just past, with a line that echoes Sophocles' Antigone: "Horrors I've witnessed aplenty ..." (8699). And Helen is the one that caused it all, hers is "the face that launched a thousand ships" (Marlowe V, i). A good-for-nothing by  the age of ten (6530). There is an echo of Diomedes' account of her in Shakespeare's Troilus and Cressida:

For every false drop in her bawdy veins
A Grecian's life hath sunk; for every scruple
Of her contaminated carrion weight
A Troyan hath been slain. Since she could speak,
She hath not given so many good words breath
As for her Greeks and Troyans suff'red death.
IV, i.

Phorkias/Mephisto rubs it in by reciting, and forcing Helena to recite with her/him, her life's story, including her double as the Egyptian Helena, and her return from Hades to live with Achilles, also returned from Hades. There is a faint hint at what might have been had she been allowed to follow her heart and marry Patroclus instead of Menelaos, who was her father's choice. Who is she now? She herself no longer knows, the total confusion causes her to faint, and we understand that only the amnesia brought about by a new beginning can restore her, if Menelaos does not get to her first.

So Helen is forced into yet another relationship not of her choosing, for her own sake to be sure but, equally important, for that of her servants. Faust, who has built a castle nearby that will instantly harbor the women, is their only hope. It's up to the queen. Helen's spontaneous "Wie sieht er aus?" ("What does he look like?")  and Phorkyas' laconic "Nicht uebel" ("Not bad") give away the real issue here. Faust in the role of savior, the knight in shining armor, like St. George, Perseus. Helena the victim held captive or about to be devoured, the woman in distress, Andromeda. Of course she is eager to be saved. But liberation has a price, the liberated herself is the reward. Helen knows her role in this encounter and accepts it, not entirely unhappily either, or so it would seem (9071, 9140).
That the new relationship becomes one of passion has many causes but, in the end, is a precious gift they receive from one another. Helen's situation instantly changes from one of rejection, degradation and imminent death to one of honor and highest esteem. Lynkeus is stunned into silence by her beauty. Faust surrenders himself and all he has and woos her with all the ardor and respect and courtly manner at his disposal. She in turn invites him to share with her all he's just given her. This exchange is a sublime game, based on self-confidence and mutual trust that none will exploit the other but will respond as generously. Even Faust's language is gentle and sonorous and seductive, and Helen is immediately at home in it. When she completes Faust's sentences, finding the word that rimes with his, we know that the attraction and surrender are mutual. Helen is won over: Da bin ich! da!  / How willingly I say: Look I am here (9412). Faust is momentarily shaken to hear it articulated with such directness, but when Helen tries to explain and differentiate he picks up the prefix: Da-sein ist Pflicht. Fate bids us Exist. Do not reflect on this unique turn of events! Relish it! (9418). That is a Greek stance, Epicurean: live in the present. Carpe diem, quam minimum credula postero (Horace). We are witnessing Faust's conversion to "her" world.

Remember what we said about two worlds merging, the medieval and the Homeric, until both are transcended. And their two representatives Faust and Helen, like every pair of lovers, begin at the beginning, in Paradise, the first world (Arcadia here). Neither has a "past" (Vergangenheit sei hinter uns getan / the past behind us and beyond recall  (9563) or better still, neither is any longer defined by the past. All lovers start all over (Der ersten Welt gehoerst du einzig an / feel yours uniquely this first land of all (9565), Faust and Helena become "just" a man and a woman and will recreate and define themselves in their new present.
But Faust can't quite shed what he has acquired in "medieval" times, and why should he as long as it is a timeless insight or image: ich bin dein, und du bist mein / I am yours, and you are mine, he says to Helen when they return from "Arcadia" (9704). This is the first line, slightly altered, of one of the most beautiful, tender and intimate Middle High German love poems (inspired no doubt by the Song of Solomon where it occurs twice, 2.16 and 6.3), charming for the very simplicity of image and diction, anonymous:

Du bist min, ich bin din
des solt du gewis sin
du bist beslozzen
in minem herzen:
verlorn ist das sluzzelin:
du muost immer drinne sin.

You are mine, I am yours
please believe that it is so
you are locked
in my heart:
lost is the key:
you'll have to stay in me.

The lovers remain out of sight for a long time. We have Phorkias' report. Their child, Euphorion, changes everything. The pair that was focused on each other now focuses, together, (although Faust is a little slower to make the adjustment) on their offspring. An everyday reality is here presented in the guise of myth. Love for the youth is accompanied and in the end overshadowed by worry and fear. The youth, on the other hand, in  the process of emancipation, will not and cannot pay heed and is killed by his very exuberance.

Paraphrasing Henry IV: Thus do our hopes touch ground and dash themselves to pieces.

Much has been written on why Euphorion is the way he is, on who he is, that he embodies the extreme of Faust's "striving" and is tempestuous, unrestrained, untempered, bold and foolhardy. That he is Faust's son and Zeus' grandson. And more, all of it true.
But poetic reality offers a simpler explanation. Faust's union with Helen must find an abrupt and yet acceptable ending, an ending not caused by age or a decline of their love. In Troilus and Cressida the two celebrated lovers, now seven years older, appear as pathetic ghosts of their former selves and are savagely deflated along with the celebrated heroes of the Trojan War.* In Faust the union is not measured in numbers of years, and the high level of energy is maintained to the end. This was the case in the Tragedy of Gretchen too. But here there's no remorse, no dreadful burden of guilt. But there is regret, an almost unbearable sadness and sense of loss in Helena's last words and body language. She follows her son to Hades. Her servants, however, remain  here and dissolve into nature's little spirits and elves.

Helena's garment becomes a cloud that ferries Faust back to the world he has known. As it fades away it assumes shapes that remind him of Juno, Leda, Helena. All three? An ideal of beauty rather than a distinct person? How is that possible? Was it a dream to be discarded at dawn? An episode that leaves no trace? Helen is never again mentioned. Then what is the "fluecht'ger Tage grosser Sinn"  / "the dazzling meaning of these fleeting days" (10054)? We have a hunch that it is more than Goethe cared to, or could, express. It was an experience complete in itself, unique, an alternate existence in an alternate reality, to be fully involved in while it lasted, and in need of no epilogue or storage in memory when it is over. In Faust's own words, it was "Dasein", a total absorption in the present, no less than a complete expenditure of all available energy.
Let's think of it as an interlude, an incarnation in another dimension, a time and place reminiscent of times and places we know, yet newly arranged in a different pattern. It is another Faust as well, and another Helena. And another life altogether. When it is spent and the previous life is resumed, no surface memory of it remains.

He is back "home" now and at ease with the memory of another woman. His first love? Gretchen? This last image does not dissolve, it rises and draws him with it: "Und zieht das Beste meines Innern mit sich fort / And draws my best and inmost soul to follow it" (10066).

I would like to conclude this with a comment offered by a friend:

"... i read the helena essay again. ... and now it's my turn to be saddened, not by what disturbed you today - a woman in pain - but by a woman forgotten.  i think i understand what you write about "Dasein," yet helen's fate here with faust is the opposite of her fate in literature.  the thing that makes her an icon to women is not her beauty - that is the underlying cause of her pain and the pain she wreaks - but her permanence, that she is remembered.  with faust she is not even that.  is her lasting effect that she points him back to gretchen?  as a figure of literature she can do that, but no woman would want that role ..."
 

* Read in this context Ivan Nagel, "Shakespeare ueber Krieg und Liebe" in MERKUR, No. 596, November 1998.