I would suggest that a number of things combine to clarify the situation. Some have been mentioned before. Briefly stated:
1. The saviour of mankind is defined as being the seed of woman (Genesis 3:15). That indicates genetic continuity and inheritance. Hence Jesus was formed from one of Mary's ova.
(The significance of a woman producing seed, as opposed to the normal concept of men producing seed, will be seen below.)
2. The prophesied coming King was to be a descendant of Judah (Genesis 49:10). That also indicates genetic continuity.
3. Jesus inherited His messiahship from his mother. According to both the Jewish law and the Roman law of the time, Jesus could not have inherited His messiahship from Joseph. (Literature references can be provided if requested (assuming I can still find them).)
Genetic continuity once again.
4. It was Adam that deliberately disobeyed God; Eve was deceived. Combine that with the seed of woman, and it becomes clear that the sinful nature of mankind, and its inherited sin, is passed down via the male line.*
5. Jesus is called the second Adam because He was the second (male) human to be formed by a direct action from God, and because He was sinless, as was Adam originally. Also, as Adam's lamentable action (deliberate disobedience) affected the whole of mankind, so did (would) Jesus' deliberate action (obedience and atoning death) similarly affect all mankind. (See 1 Corinthians 15:20-22 for the general statement.)
20 But now is Christ risen from the dead, and become the firstfruits of them that slept.
21 For since by man came death, by man came also the resurrection of the dead.
22 For as in Adam all die, even so in Christ shall all be made alive.
If the above thoughts have credence, it would seem that there is no great mystery here after all.
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* The doctrine of the “Immaculate Conception”, was not officially defined until 1854, when it was declared by Pope Pius IX in his papal bull Ineffabilis Deus. (If my reference is correct.)
The doctrine was the result of theologians wrestling with the perceived problem of how Jesus could have avoided inheriting original sin from his mother.
Neither Jesus nor his apostles betrayed any awareness of that idea, nor the need for it.
In fact, it would seem that the concept was unknown before the 5th century, 400 years after Jesus walked the Earth.
What was America like 400 years ago? Or England? Or Australia, etc.?
400 years is a long time for a doctrine not to be needed.