Or fled before . . Now swift, fierce & obscene
The wild dance maddens in the van, & those
Who lead it, fleet as shadows on the green,
Outspeed the chariot & without repose
Mix with each other in tempestuous measure
To savage music

It is only in context that this passage of Shelley's dream vision are to be understood. We can see that the four faced charioteer ("four faces of that charioteer") drove "a captive multitude" before the chariot. We are told of what sorts of people this multitude is comprised, such as aged rulers or aged sufferers (odd juxtaposition ...), those who suffer to the last moment of their lives, those with either fame or infamy, etc. We are then told of those who do not comprise the captive multitude. These are specifically "the sacred few" who being born, die immediately and "like eagles" flee back to whence they came:

As they had touched the world with living flame
Fled back like eagles to their native noon,

It is these who are identified as being "they of Athens & Jerusalem" who are neither among those who comprise the captives nor those mocked the captives. In summary, the passage you ask about identifies the "sacred few" who are neither captive nor mockers of the captives; mockers "whose unholy leisure / Was soothed by mischief since the world begun, ...." The metaphor of "sacred few" relates to those who die as infants; the metaphor of "mockers" relates to the wicked of Earth, or even wicked demons.

In "The Triumph of Life," Shelley shows us how, throughout history, the human spirit has often been crushed by life itself. In ancient Rome, triumphs were large, lavish processions staged to celebrate a military victory. They were elaborate affairs, with long lines of captive slaves from the vanquished enemy dragged along in their wake. This is an appropriate metaphor for how Shelley sees life itself. Life triumphs over the human spirit just as completely as the Roman legions triumphed over their enemies. For Shelley, life is a "painted veil" that conceals more than it reveals. Like a Roman triumph, it is a colorful pageant which successfully disguises its crushing of the human spirit. The crowds that throng the triumph's route may cheer, just as we may derive a superficial enjoyment of life. But their spirits—and ours—have been crushed just as surely as the sad procession of vanquished slaves paraded during a Roman triumph.

In Percy Bysshe Shelley's "The Triumph of Life," there are several similes used.

Around line 290, Shelley writes about the Church rising "like shadows" between man and God. I take this to mean that Shelley feels that instead of drawing mankind closer to God, the Church, specifically Pope Gregory the Great, is acting more like an obstruction.

The recurring theme here seems to speak to the conflicts organized religion (Christianity) has created that separates mankind from God.

Then there is a shift at approximately line 314: Shelley is describing the coming of spring; the speaker notes he had fallen asleep under a mountain as the world outside comes to life. He was in a "cavern deep," there for an immeasurable time, and...

...from it came a gentle rivulet / Whose water like clear air in its calm sweep...

This simile draws attention to the beauty of all that is natural, that surrounds him, but there is a sense of the mystical: we find this in...

I found myself asleep
Under a mountain, which from unknown time
Had yawned into a cavern high and deep

We see it again with the following:

And from it came a gentle rivulet
Whose water like clear air in its calm sweep 315
“Bent the soft grass and kept for ever wet /
The stems of the sweet flowers, and filled the grove
With sound which all who hear must needs forget...

The sense of the "supernatural" (not in sense of ghosts, etc., but things beyond the "natural world") would have been anything but unusual in the poetry of Shelley, one of the three great second-generation Romantic poets in English literature.

In this mystical place, Shelley describes the water "like clear air," and sounds like an allusion to Greek Mythology and the the River Lethe:

...it was believed that the newly dead who drank from the River Lethe would lose all memory of their past existence.

So that the water in the poem, like the sweet air Shelley describes:

With sound which all who hear must needs forget
“All pleasure and all pain, all hate and love, Which they had known before that hour of rest: 320
A sleeping mother then would dream not of
“The only child who died upon her breast At eventide, a king would mourn no more
The crown of which his brow was dispossest...

Shelley describes that this water would remove all memory previous, and this included the memory a mother would have of losing her only baby, but would also comfort the king who had lost his crown and kingdom.

Lines 469-80 of Percy Bysshe Shelley’s poem “The Triumph of Life” are particularly important, because these lines allude to the great Italian poet Dante. Dante, many centuries earlier, had written The Divine Comedy, a precursor of Shelley’s poem not only in its use of terza rime (consisting of three-line stanzas rhyming aba, bcb, cdc, and so on) but also in many of its themes. The relevant passage reads as follows:

469  "Before the chariot had begun to climb
470   The opposing steep of that mysterious dell,
471   Behold a wonder worthy of the rhyme
472   "Of him whom from the lowest depths of Hell
473   Through every Paradise & through all glory
474    Love led serene, & who returned to tell
475   "In words of hate & awe the wondrous story
476   How all things are transfigured, except Love;
477   For deaf as is a sea which wrath makes hoary
478   "The world can hear not the sweet notes that move
479   The sphere whose light is melody to lovers---
480   A wonder worthy of his rhyme----

Lines 472-74 refer to Dante being led through the inferno (Hell), then through Purgatory, and finally into Paradise. Lines 474-80 explain that everything except Love is mutable (“all things are transfigured”). Humans can no longer hear the “music of the spheres” associated with Love. Such music was “worthy of [Dante’s] rhyme.

Shelley thus associates himself with his great medieval predecessor. He hopes to produce a poem as notable as Dante’s famous epic. Dante is one of several important precursors whose influence on the poem is obvious. Others include Petrarch, Milton, Wordsworth, Lucretius, and Plato (see the introduction to the poem in the Norton edition of Shelley’s Poetry and Prose, edited by Donald H. Reiman and Neil Fraistat). Jean Jacques Rousseau is an actual figure in the poem, performing the same role for Shelley’s speaker that the Roman poet Virgil performed for Dante during the latter’s journey through the Inferno.

It was typical of epic poets to associate themselves with their great predecessors.  Thus Virgil echoed Homer; Dante echoed Virgil; Spenser echoed Virgil and numerous Italians who were influenced by Virgil; and Milton echoed Homer, Virgil, Dante, Spenser, and numerous other writers besides. Shelley, then, is following in a long and lofty tradition of poets alluding to previous masters of the art.

In Percy Bysshe Shelley’s poem “The Triumph of Life,” a youth observes a ghostly procession of various specimens of humanity. He asks the spirit of Jean Jacques Rousseau to explain the procession to him.  Rousseau himself at one point alludes to Dante’s poem The Divine Comedy, paraphrasing part of the meaning of that poem by asserting that

“The world can hear not the sweet notes that move
The sphere whose light is melody to lovers --  . . .” (478-79)

The Norton Anthology of English Poetry (6th edition) explains the reference to “The sphere” as follows: “The third sphere of the planet Venus (Love), in Dante’s Ptolemaic universe.”

Essentially, Shelley’s lines suggest that true love is lofty, transcendent, elevated, and sublime. People who are caught up in the world – whose minds are fixated on material things and possessions – cannot hear the pleasing, heavenly music of love, which is described as a kind of light.

Reactions to Shelley’s lines have been various and have included the following:

• Evelyn Underhill saw the lines as evidence of Shelley’s mysticism.
• Edward Duffy read the lines as evidence of the beauty of love but also of much of mankind’s inability to appreciate or even perceive that beauty.
• David Wallace and many others interpreted the lines as evidence of Shelley’s very strong interest in Dante.
• Carleton W. Stanley saw the lines as evidence of Shelley’s debt to Plato and also of Shelley’s sympathy with Platonic thinking.
• Glenn O’Malley discussed the lines in connection with a broader discussion of Shelley and synaesthesia (for example, the ability to “hear” light or “see” music).
• Other such comments can be found in this Google Books link:

The general tone of "The Triumph of Life" is despairing, if not completely despondent. Shelley presents us with a view of human history in which a succession of noble spirits has been crushed beneath the inexorable Chariot of Life. At the same time, he defiantly exalts those like him, the "sacred few" who have not compromised with the values and fleeting fancies of earthly life. Even in the gloomiest lines of the poem, there still remains a characteristic spark of the old Shelleyan élan and vigor, which hints at the possibility of a bright new beginning that transcends the temporal world and all its disappointments.

In the passage referred to in the question, the philosopher Rousseau has been seized by a new vision after drinking deeply from the cup of knowledge. Now he can see life more clearly, can see it for what it really is: a grotesque pageant of phantoms and shadows in which we dance insanely and unthinkingly in the wake of the Chariot of Life. Previously, Rousseau had been blinded by the light of his own philosophy in seeing nature as pristine and incorruptible. Yet after drinking of the cup of knowledge, Rousseau sees the truth: that it is the very natural life he previously extolled and romanticized that corrupts and triumphs over the human spirit.

In lines 410-423 of Shelley's poem "Triumph of Life," Shelley is examining the light of earth which is important to survival. For one to end the day in the same way they began it is important. Beginning a day with the light of hope allows one to have a positive outlook on what is to come. Ending a day with the same light allows one to embrace the promise of hope again.

Shelley also refers to a dream, a lament (an expression of grief). within a dream, one loses the light one needs to see the promise of hope: the "light of heaven, whose half-extinguished beam" fails to provide the comfort of light one sees when awake.

In the end, Shelley is stating that it is the light which people seek so that they never become lost.

[Poetry is, like all art, something that takes on new life in the face of each person who interacts with it, based on each individual's life experiences. There are many ways to interpret art, though it is—in my mind—almost impossible to say one interpretation is "the only" interpretation. The following is from research, as well as my own personal interpretations.]

In Shelley's "The Triumph of Life," lines 211-247, I did not find any similes. However, I can provide an overview of what Shelley seems to be saying in this segment of the poem.

Shelley begins this section referring to the great ones who have come before: leaders of the church, military and kingdoms, intellectuals ("mitres, helms, crowns, and haloed sages"). Shelley believes that with all the knowledge these great individuals were given, they were never instructed in the art: "to know themselves."

These great ones hid their ideas as one would hide mutinous thoughts (thoughts that did not conform to the popular view) under the cover of darkness (perhaps symbolizing the lack of knowledge), rather than out in plain sight (the light, symbolic of knowledge or enlightenment).

Shelley refers to the French writer and philosopher Rousseau, who though not a contemporary of Shelley's, had an enormous effect upon the world and how "men" thought. Regarding "The Child of the fierce hour:"

The sparks of Rousseau’s writings had kindled a thousand signal fires—including that of the French Revolution, of which one child was Napoleon, who is described in lines 215–27.

Shelley seems to feel that powerful "intellects" have crippled the world, hindering advancement—enabling anarchists to rise, while hampering great minds:

Presumably Voltaire (the immensely influential thinker of the 18th-century Enlightenment) is the “dem- agogue.” Frederick the Great of Prussia, Catherine the Great of Russia, and Leopold II of the Holy Roman Empire, all influenced by Voltaire’s ideas, are the “anarchs” (leaders who bring about anarchy). Immanual Kant (the great philosopher of the German Enlightenment) is the “sage.”

Overall, at the end of the passage, Shelley seems to indicate that the passionate nature and ideas of Rousseau prohibited the philosopher from ever being truly happy, as he strove toward impossible ideals. My sense is that Shelley, after the time of Rousseau and other "influential minds," looks not to the significance or prominence that may have been "falsely" attributed to those who followed them, but those who came before—"the old faded"—who one might believe knew a truth that has been obscured from the world.

In lines 295 through 305, the poem’s speaker is continuing to converse with the spirit of the philosopher Jean-Jacques Rousseau, who is serving as his “leader” or guide. In the preceding lines, the speaker has identified numerous “conquerors” and holy men, such as the Catholic popes Gregory and John, and claimed they did more to obscure than to illuminate true spiritual understanding that he refers to as “the Sun.” In contrast to those men, who had destructive power, Rousseau’s spirit says that he had the power to create. However, this creation may be filled with pain, or “a world of agony" (Line 295).

In line 296, the speaker asks Rousseau about his origins and destination. Rousseau initially offers a vague reference to feeling “sick,” because he is constantly surrounded by people and saddened by a single thought. He says that he only knows “[w]hence [he]…came, partly” and has a good idea how and why he arrived at his current difficult situation; he uses the metaphor of traveling along paths to a “pass,” or route between mountains. This leader also thinks that the speaker can guess how this occurred. However, he cannot figure out the reasons behind these events, or where he is going next (lines 303–304):

Why this should be my mind can compass not;
"Whither the conqueror hurries me still less.

Rousseau urges the speaker to follow him and thus become either “an actor or a victim in this wretchedness….” Rousseau thinks that the speaker can then teach him what he has learned (lines 307 – 308). From this point, he tells a long story by way of answering the speaker’s question.

This format does not accommodate a prose paraphrase of the entire selection from line 210 to 309, but I can render a portion of the selection into a prose paraphrase upon which you might draw to paraphrase the rest. I'll start at line 268 and end at 295:

"The jealous keys of truth's eternal doors

"If it be but a world of agony."--

PROSE PARAPHRASE:

"Eternal truths would have still been kept away as though locked by eternal doors if Bacon had not entered into philosophical discussion and unbarred the caves of thought and revealed the treasures of wisdom that even poets quell with their poetic song, which gives the strain of living truth, if truth be thought of as a contagion to the vein that causes authors to suffer vile pain, since words of truth are seeds of misery," said the leader speaking in the dream vision.

"My words were seeds of misery even as the deeds of others," the leader said, pointing to Caesars up to Constantine--all laden with crimes--and to Anarchs who murder and force into being ruling lineages that spread plague and blood and the greed for gold abroad. He pointed also to Popes Gregory and John, men divine, who rose up to separate man from God through teaching shadows (not truths) that overpowered and quenched the true Sun. "Their power," he said, "was given but to destroy--to create but a world of agony."