Since it doesn't matter how we are created, so far as sinning or not, then why do you insist that Christ now creates us differently than Adam?
And where does the Bible say He does?
Adam was made from the dust of the earth (Genesis 3:19) - he was not born as a baby. Eve was made from one of Adam's ribs - she was made as a woman, not a baby. But everyone else has been made by humans procreating, and are born as babies. So clearly Adam was made differently from everybody else.
It does matter how we are created, so far as sinning is concerned. Adam was created perfect and could have remained sinless, just as God prepared a perfect human body for Jesus and Jesus remained sinless. After Adam and Eve had sinned they were changed. Eve, and all women descendants, would suffer much greater pain in childbirth (unlike all other mammals which don't suffer anywhere near as much as women do). Men were condemned to toil for food, and he and Eve would also suffer dying and death (by being barred from the food that would enable them to live forever). So their bodies and minds started decaying, until they eventually died. All their offspring were produced during that dying process, so everyone was born less than perfect physically and mentally, and even more inclined to sin than Adam.
Now you know if he repented or not.
Irrelevant. God had not at that time revealed any way of escape from the penalty of death. Whether Adam repented or not, it would not make any difference - he would still have died.
Who is it imputing sin into the seed and flesh of man? Who made this sin 'spirit' or 'gene' to impute in the first place?
I've already covered that. Romans 5:12 (WEB):
(12) Therefore as sin entered into the world through one man, and death through sin; so death passed to all men, because all sinned.
Sin entered the world, that is all humankind, through Adam. So all humankind have become sinful. The fact that all men (people) have died shows that everyone has sinned. Barnes' Bible notes says:
Sin entered into the world - He was the first sinner of the race. The word “sin” here evidently means the violation of the Law of God. He was the first sinner among people, and in consequence all others became sinners. ... He says, therefore, that Adam was the first sinner of the race, and that death was the consequence.
Into the world - Among mankind; Joh_1:10; Joh_3:16-17. The term “world” is often thus used to denote human beings, the race, the human family. The apostle here evidently is not discussing the doctrine of original sin, but he is stating a simple fact, intelligible to all: “The first man violated the Law of God, and, in this way, sin was introduced among human beings.” In this fact - this general, simple declaration - there is no mystery.
'Sin' dies not pass upon anyone by 'inheritance' made in the womb.
So how do you explain the death of babies during or shortly after birth? Why do they suffer the wages of sin - death? If a baby hasn't sinned, but has a sinful nature, then is God justified by applying/allowing the penalty for sin (death)? Again, Barnes' Bible notes says:
2. Moreover, there are certain facts connected with the moral history of mankind, which present insuperable difficulties, if we deny the doctrines of representation and imputed sin. “How shall we on any other principle account for the universality of death, or rather of penal evil?” It can be traced back beyond all personal guilt. Its origin is higher. Antecedent to all actual transgression, man is visited with penal evil. He comes into the world under a necessity of dying. His whole constitution is disordered. His body and his mind bear on them the marks of a blighting curse. It is impossible on any theory to deny this. And why is man thus visited? Can the righteous God punish where there is no guilt? We must take one side or other of the alternative, that God inflicts punishment without guilt, or that Adam’s sin is imputed to his posterity. If we take the latter branch of the alternative, we are furnished with the ground of the divine procedure, and freed from many difficulties that press upon the opposite view.
It may be noticed in this place also, that the death of infants is a striking proof of the infliction of penal evil, prior to personal or actual sin. Their tender bodies are assailed in a multitude of instances by acute and violent diseases, that call for our sympathy the more that the sufferers cannot disclose or communicate the source of their agony. They labor with death and struggle hard in his hands, until they resign the gift of life they had retained for so short a while. It is said, indeed, that the case of infants is not introduced in Scripture in connection with this subject, and our author tells us, that they are not at all referred to in any part of this disputed passage, nor included in the clause, “death reigned, even over them that had not sinned after the similitude of Adam’s transgression.” On this, some observations will be found in the proper place. Meanwhile, there is the fact itself, and with it we are concerned now. “Why do infants die?” Perhaps it will be said that though they have committed no actual sin, yet they have a depraved nature; but this cedes the whole question, for that depraved nature is just a part of the penal evil, formerly noticed. Why are innocent infants visited with what entails death on them? One answer only can be given, and no ingenuity can evade the conclusion, “in Adam all die.” The wonder is, that this doctrine should ever have been denied. On the human family at large, on man and woman, on infant child, and hoary sire, on earth and sky, are traced the dismal effects of the first sin.
So Barnes implies that it is God that imputes sin on everybody, and that because of Adam's sin everybody dies.