1. Clearly human form is added when Jesus PBUH came to this world. If God was perfect then there is no need for addition or subtraction from a perfection.
Jesus was perfect in His human form. He had all His powers.
2. Lets focus on the word SON of God. You said God is all one, the father, the son and holy spirit. God is also eternal. Who came first son or father and how they can be eternal?
Catholics believe that both has always been.
3. Now lets see how you justify the concept of incarnated. My point one above pretty much refutes this concept. But lets assume for a while that God did incarnate. By definition, God is infinite. You can't box God in human body, otherwise He is not infinite. You can't make God dependent on his creation like Jesus PBUH did on his mother, on the food and other things which are creation of God. God can't be seen and yet many saw Jesus PBUH. God knows all, yet Jesus PBUH had no knowledge of the hour. God is all powerful and does not need anyone's help, yet Jesus PBUH prayed to God to remove the cup from him.
Jesus was conceive at conception, by the Holy Spirit (not Gabriel).God lowered Himself by becoming one of us to save us.
The Lord’s descent into hell
“What is happening? Today there is a great silence over the earth, a great silence, and stillness, a great silence because the King sleeps; the earth was in terror and was still, because God slept in the flesh and raised up those who were sleeping from the ages. God has died in the flesh, and the underworld has trembled.
Truly he goes to seek out our first parent like a lost sheep; he wishes to visit those who sit in darkness and in the shadow of death. He goes to free the prisoner Adam and his fellow-prisoner Eve from their pains, he who is God, and Adam’s son.
The Lord goes in to them holding his victorious weapon, his cross. When Adam, the first created man, sees him, he strikes his breast in terror and calls out to all: ‘My Lord be with you all.’ And Christ in reply says to Adam: ‘And with your spirit.’ And grasping his hand he raises him up, saying: ‘Awake, O sleeper, and arise from the dead, and Christ shall give you light.
‘I am your God, who for your sake became your son, who for you and your descendants now speak and command with authority those in prison: Come forth, and those in darkness: Have light, and those who sleep: Rise.
‘I command you: Awake, sleeper, I have not made you to be held a prisoner in the underworld. Arise from the dead; I am the life of the dead. Arise, O man, work of my hands, arise, you who were fashioned in my image. Rise, let us go hence; for you in me and I in you, together we are one undivided person.
‘For you, I your God became your son; for you, I the Master took on your form; that of slave; for you, I who am above the heavens came on earth and under the earth; for you, man, I became as a man without help, free among the dead; for you, who left a garden, I was handed over to Jews from a garden and crucified in a garden.
‘Look at the spittle on my face, which I received because of you, in order to restore you to that first divine inbreathing at creation. See the blows on my cheeks, which I accepted in order to refashion your distorted form to my own image.
‘See the scourging of my back, which I accepted in order to disperse the load of your sins which was laid upon your back. See my hands nailed to the tree for a good purpose, for you, who stretched out your hand to the tree for an evil one.
`I slept on the cross and a sword pierced my side, for you, who slept in paradise and brought forth Eve from your side. My side healed the pain of your side; my sleep will release you from your sleep in Hades; my sword has checked the sword which was turned against you.
‘But arise, let us go hence. The enemy brought you out of the land of paradise; I will reinstate you, no longer in paradise, but on the throne of heaven. I denied you the tree of life, which was a figure, but now I myself am united to you, I who am life. I posted the cherubim to guard you as they would slaves; now I make the cherubim worship you as they would God.
“The cherubim throne has been prepared, the bearers are ready and waiting, the bridal chamber is in order, the food is provided, the everlasting houses and rooms are in readiness; the treasures of good things have been opened; the kingdom of heaven has been prepared before the ages.”
Source: Ancient Homily for Holy Saturday: PG 43, 440A, 452C: LH, Holy Saturday, OR.
Source approved by the Vatican of the Roman Catholic Church.
How do we know what Paul believe when we are uncertain who wrote the books of the New Testament. It is believe that Paul wrote most of it.