At what point in time and creation did Satan (Lucifer the former arch angel) along with all the other angels who followed him fall from God's glory and caste out of the heavenly realm? Also which act is it that they (Lucifer and the angels who follow him) do that makes God condemn them from the glory and reign of heaven. I know in the garden of Eden he was condemned as a serpent but had he already been a fallen angel by then or was it not until later, possibly when "the sons of God" sought after the daughters of man and created the Nephilim in Gen. 6 that they were cast out?
The Bible does not give a specific account of when Satan and the fallen Angels were ejected from Heaven, but we are given some clues as to a timeframe.
All Scripture is quoted from the King James translation.
Isaiah 14:12-14 KJV
How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations! For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north: I will ascend above the heights of the clouds; I will be like the most High.
Isaiah chapter 14 tells us why , and gives us a clue as to when. Lucifer tried to usurp the position of God, and take over Heaven and that is the reason. The clue comes in the being cast down to the ground. If Satan was cast to the ground it had to be after:
Genesis 1:9 KJV
And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so.
until the third day there was no dry ground.
Jesus gives us another clue in:
Luke 10:18 KJV
And he said unto them, I beheld Satan as lightning fall from heaven.
Jesus tells us he was present and that Lucifer and Satan are one and the same.
The rest we get in the Revelation:
Revelation 12:3-4 KJV
And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads. And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
The red Dragon here refers to Satan and the part about his tail casting one third of the stars to the ground refers to one third of the Angels rebelling with Lucifer and being cast to the Earth with him.
Lastly as you stated in your question Satan was behind the serpent tempting Eve.
So using those clues we can only determine that they were cast out of Heaven after the second day of creation and before the fall of mankind.
The bible does not provide a specific date for creation. Ussher Chronology proposes 4004BC. Angels were created before the earth and man.
Job 38:7 KJV
When the morning stars sang together, and all the sons of God shouted for joy?
Lucifer was a covering cherub.
Ezekiel 28:7 KJV
Behold, therefore I will bring strangers upon thee, the terrible of the nations: and they shall draw their swords against the beauty of thy wisdom, and they shall defile thy brightness.
Lucifer's desire for self-exhaultation led to rebellion against God. He seduced a third of the angels to follow in his rebellion.
Isaiah 14:12-13 KJV
How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations!
For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
Ezekiel 28:15-18 KJV
Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.
By the multitude of thy merchandise they have filled the midst of thee with violence, and thou hast sinned: therefore I will cast thee as profane out of the mountain of God: and I will destroy thee, O covering cherub, from the midst of the stones of fire.
Thine heart was lifted up because of thy beauty, thou hast corrupted thy wisdom by reason of thy brightness: I will cast thee to the ground, I will lay thee before kings, that they may behold thee.
Thou hast defiled thy sanctuaries by the multitude of thine iniquities, by the iniquity of thy traffick; therefore will I bring forth a fire from the midst of thee, it shall devour thee, and I will bring thee to ashes upon the earth in the sight of all them that behold thee.
Revelation 12:4 KJV
And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
Lucifer was condemned before the fall of man. The serpent was the medium through which deceit was practiced. Sons of God and daughters of men. The phrase sons of God depict the righteous.
Hosea 1;10 KJV
Yet the number of the children of Israel shall be as the sand of the sea, which cannot be measured nor numbered; and it shall come to pass, that in the place where it was said unto them, Ye are not my people, there it shall be said unto them, Ye are the sons of the living God.
John 1:12-13 KJV
But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name:
Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
Romans 8;14 KJV
For as many as are led by the Spirit of God, they are the sons of God.
Daughters of men depict the wicked.
Ecclesiaste 8:11 KJV
Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil.
The premise that the sons of God are fallen angels is error. Further, angels do not procreate.
Genesis 6:2 KJV
That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
does not mention giants. However,
Genesis 6:4 KJV
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Obviously, there were giants in the earth before the sons of God.
The Fall of Lucifer happened before the creation of Man, furthermore He is indeed a creation of God "And spared for our Judgment"
"The Father Judges No one yet passes all Judgment to the Son" (Yeshua) or in vulgar tongue Jesus Christ.
The Earth existed before light was shown upon it in the Void.
Mankind also has more than Adam and Eve in its history according to Genesis, read it again and again focus on the story of Cain it is the pointed Argument that led to the teaching of evolution in schools.
Lucifer is so charged by God to serve, "The wicked shall be made into the footstool of the Righteous"
does a footstool serve a purpose? Yes!