All the time too that the child is in the womb of its mother, it receives all its life and nourishment from outside of itself; if it were cut off from that life, it would be in a dead state; so it is with the soul here, if it is cut off from its spiritual food, it is dead.
(‘Abdu’l-Bahá, Bahá’í Prayers 9, p. 47-48)

As it is not yet shown while the child is in the womb of its mother, what its condition will be, whether it will have all the gifts of God or not, whether it will be perfect in all its members or not, whether it will be blind, or deaf, or dumb—but afterwards, when it enters the world, then it becomes clearly apparent if it is defective or not—so it is with the soul in this present state. Its perfection or its lackness is not understood until it enters the heavenly kingdom; then it is clearly seen, and then the soul understands whether or not it is lacking in the gifts of God.
(‘Abdu’l-Bahá, Bahá’í Prayers 9, p. 47)

As the child in the womb does not yet know the use of its members, it does not know what its eyes are for, neither its nose, nor ears, nor tongue—so also it is with the soul on earth. It cannot understand here the uses and powers of its spiritual gifts, but directly it enters the eternal kingdom, it will become clearly apparent.
(‘Abdu’l-Bahá, Bahá’í Prayers 9, p. 48)

But with the human soul, there is no decline. Its only movement is towards perfection; growth and progress alone constitute the motion of the soul.
(‘Abdu’l-Bahá, Paris Talks, p. 89)

Consider again the sun when it is completely hidden behind the clouds. Though the earth is still illumined with its light, yet the measure of light which it receiveth is considerably reduced. Not until the clouds have dispersed, can the sun shine again in the plenitude of its glory. Neither the presence of the cloud nor its absence can, in any way, affect the inherent splendor of the sun. The soul of man is the sun by which his body is illumined, and from which it draweth its sustenance, and should be so regarded.
(Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 154)

Consider how the human intellect develops and weakens, and may at times come to naught, whereas the soul changeth not. For the mind to manifest itself, the human body must be whole; and a sound mind cannot be but in a sound body, whereas the soul dependeth not upon the body. It is through the power of the soul that the mind comprehendeth, imagineth and exerteth its influence, whilst the soul is a power that is free.
(‘Abdu’l-Bahá, )

Divine perfection is infinite, therefore the progress of the soul is also infinite. From the very birth of a human being the soul progresses, the intellect grows and knowledge increases. When the body dies the soul lives on. All the differing degrees of created physical beings are limited, but the soul is limitless!
(‘Abdu’l-Bahá, Paris Talks, p. 89)

For, verily, the signs of these triune powers which exist in mankind are spirit, mind and soul. The spirit is the power of life, the mind is the power which apprehendeth the reality of things, and the soul is an intermediary between the Supreme Concourse and the lower concourse.
(Bahá’í Scriptures, p. 464)

If it chooses the material world as a partner, then the child born of that union will be a materialistic way of life which deprives the soul of its spiritual heritage. A great many people in the world allow themselves to fall in love with material things; consequently the soul is impoverished and although it is a spiritual entity, it becomes sullied with worldly affections and gives birth to materialism, an offspring unworthy of its high station. But the Covenant of God enjoins upon man to recognize His Manifestation and turn to Him.
(Adib Taherzadeh, The Covenant of Bahá’u’lláh, p. 17)

In all religions the belief exists that the soul survives the death of the body. Intercessions are sent up for the beloved dead, prayers are said for their progress and for the forgiveness of their sins. If the soul perished with the body all this would have no meaning. Further, if it were not possible for the soul to advance towards perfection after it had been released from the body, of what avail are all these loving prayers, of devotion?
(‘Abdu’l-Bahá, Paris Talks, p. 89)

Know thou that the soul of man is exalted above, and is independent of all infirmities of body or mind. That a sick person showeth signs of weakness is due to the hindrances that interpose themselves between his soul and his body, for the soul remaineth unaffected by any bodily ailments.
(Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 153-154)

Know, verily, that the soul is a sign of God, a heavenly gem whose reality the most learned of men hath failed to grasp, and whose mystery no mind, however acute, can ever hope to unravel.
(Bahá’u’lláh, Gleanings)

Now let us consider the soul. We have seen that movement is essential to existence; nothing that has life is without motion. All creation, whether of the mineral, vegetable or animal kingdom, is compelled to obey the law of motion; it must either ascend or descend.
(‘Abdu’l-Bahá, Paris Talks, p. 89)

Have ye forgotten that true and radiant morn, when in those hallowed and blessed surroundings ye were all gathered in My presence beneath the shade of the tree of life, which is planted in the all-glorious paradise? Awe-struck ye listened as I gave utterance to these three most holy words: O friends! Prefer not your will to Mine, never desire that which I have not desired for you, and approach Me not with lifeless hearts, defiled with worldly desires and cravings. Would ye but sanctify your souls, ye would at this present hour recall that place and those surroundings, and the truth of My utterance should be made evident unto all of you.
(Bahá’u’lláh, The Persian Hidden Words #19)

Regarding your question concerning the passage in ‘Seven Valleys’ referring to pre-existence. This in no way presupposes the existence of the individual soul before conception. The term has not been absolutely accurately translated, and what is meant is that man’s soul is the repository of the ancient, Divine mysteries of God.
(Shoghi Effendi, Lights of Guidance, p. 500)

Scientific philosophy has demonstrated that a simple element (’simple’ meaning ‘not composed‘) is indestructible, eternal. The soul, not being a composition of elements, is, in character, as a simple element, and therefore cannot cease to exist. The soul, being of that one indivisible substance, can suffer neither disintegration nor destruction, therefore there is no reason for its coming to an end.
(‘Abdu’l-Bahá, Paris Talks, p. 90)

Second, the rational soul, meaning the human spirit, does not descend into the body—that is to say, it does not enter it, for descent and entrance are characteristics of bodies, and the rational soul is exempt from this. The spirit never entered this body, so in quitting it, it will not be in need of an abiding-place: no, the spirit is connected with the body, as this light is with this mirror. When the mirror is clear and perfect, the light of the lamp will be apparent in it, and when the mirror becomes covered with dust or breaks, the light will disappear.
(‘Abdu’l-Bahá, Some Answered Questions, p. 238)

The Prophets, unlike us, are pre-existent. The Soul of Christ existed in the spiritual world before His birth in this world. We cannot imagine what that world is like, so words are inadequate to picture His state of being.
(Shoghi Effendi, Lights of Guidance, p. 504)

The human spirit which distinguishes man from the animal is the rational soul and these two names—the human spirit and the rational soul—designate one thing.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 316)

The same is true of the soul. It comes into being at the time of conception, it gradually acquires divine qualities, but there comes a time when it has to produce its fruit. Not until the soul reaches this point can it be said to have fulfilled its destiny. This can happen when, following the above principle of male and female interaction, the soul assumes the function of the female and establishes a spiritual intercourse with another agency.
(Adib Taherzadeh, The Covenant of Bahá’u’lláh, p. 17)

The soul hath two phases: the higher aspireth to the Kingdom of El-Bahá, and the lights of the mind shine forth from that horizon unto its higher sphere; the other phase inclineth to the lower concourse of the material world and its lowest sphere is enveloped in the darkness of ignorance. But when light is poured upon this phase, and if this phase of the soul is capable of receiving it, then “truth hath come and falsehood vanisheth, for falsehood is of short duration”—otherwise, darkness will surround it from all directions and it will be deprived of association with the Supreme Concourse and will remain in the lowest depths.
(Bahá’í Scriptures, p. 464)

The soul is not a combination of elements, it is not composed of many atoms, it is of one indivisible substance and therefore eternal. It is entirely out of the order of the physical creation; it is immortal!
(‘Abdu’l-Bahá, Paris Talks)

The soul, like the intellect, is an abstraction. Intelligence does not partake of the quality of space, though it is related to man’s brain. The intellect resides there, but not materially. Search in the brain you will not find the intellect. In the same way though the soul is a resident of the body it is not to be found in the body.
(‘Abdu’l-Bahá, Divine Philosophy, p. 127)

The soul or spirit of the individual comes into being with the conception of his physical body.
(Shoghi Effendi, Lights of Guidance, p. 504)

The soul pervadeth throughout the whole body, and its commands are effective in all the parts and limbs of man. Notwithstanding its utmost sanctification (or abstraction) this soul is manifest and evident in all its grades, in this material form.
(‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v1, p. 203)

The soul, like the intellect, is an abstraction. Intelligence does not partake of the quality of space, though it is related to man’s brain. The intellect resides there, but not materially. Search in the brain you will not find the intellect. In the same way though the soul is the resident of the body, it is not to be found in the body.
(‘Abdu’l-Bahá, )

This spirit, which in the terminology of the philosophers is the rational soul, embraces all beings, and as far as human ability permits discovers the realities of things and becomes cognizant of their peculiarities and effects, and of the qualities and properties of beings.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 316)

We read in the sacred writings that ‘all good works are found again’ [ i.e.—All good actions bring their own reward]. Now, if the soul did not survive, this also would mean nothing!
(‘Abdu’l-Bahá, Paris Talks, p. 89-90)

When man dies, his relation with the body ceases. The sun is reflected in the mirror; the mirror reflects the light and brilliancy of the sun, but the sun does not reside in the mirror. It does not enter nor come out of the mirror, nevertheless one sees it in the mirror, so the soul reflects itself in the body. If the mirror be broken the sun does not die. The body is the temporary mirror; the spiritual soul suffers no change, no more than the sun does remaining eternally in its own station. Even as in the world of dreams when all the physical faculties are in abeyance and the soul travels in all realms seeing, hearing, speaking, so when the physical body decomposes, the soul is not affected.
(‘Abdu’l-Bahá, Divine Philosophy, p. 127)

… for the soul is prone to evil.
(Bahá’u’lláh, The Summons of the Lord of Hosts, p. 199)