The term 'one man' appears so many times is an effort to emphasise the pivotal turns humanity took at the response of a representative man to God.
One man who faltered at a simple deception and committed all who descended from him to be smitten with the corruption of sin and estrangement from God.
The other man, who also began without sin, who also had his own will which sometimes differed from the Father, but was always brought into subjection - not of his own will and power alone, but by the presence of God in him, and his willing, humble obedience that was gained from suffering, making him perfect for the task ahead - to be the one perfect sacrifice for all. Thus reuniting all mankind with their creator again - eternally.
One Adam, the second and last Adam, undid the trouble the first Adam had begun. Both were sons of God.
Luke 3:38 The son of Enos, the son of Seth, the son of Adam, the son of God.
Luke 1:35 And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore, the child to be born will be called holy—the Son of God.
This is a profound mark of Godly wisdom, love, authority and sovereignty that He would address the impending doom of the first Adam, with a second - not with brute force of a God/man, but a human who could be tempted, who could sin, who could not win against evil on his own, but only in submission to holiness and good and live in the power of God's spirit to bring about the victory that counted.
Through death, he accomplished what the first Adam never could.
Heb 2:14 by his death he might break the power of him who holds the power of death
Jesus is said to be slain from the foundation of the world (Rev 13:8) showing that the second Adam was always the plan.
One man succumbed to evil, the other man did not. God needed the man Jesus to do this, and Jesus needed his God to succeed.
Paul in Rom 5 has taken up the challenge to express quite firmly the manner by which God has and would construct salvation and the resultant eternal life - by a man! One man intended to usher in a life of sin and pain and death. By experiencing the horrors of such a choice, mankind would be able, with God's grace and the other man, Jesus, and his holy sacrifice, be able to enter eternity without a chance of choosing sin ever again.
Rom 5:17 points out the importance of sins invincible opponent - grace, and how the grace of God will always trump sin - as if we might think they were somehow equal, Paul assures us otherwise! The grace delivered through the man Jesus, was more than sufficient to trump sin of all for all time.
Romans 6:23 For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
Ephesians 2:8 For it is by grace you have been saved through faith, and this not from yourselves; it is the gift of God
Hebrews 9:15 Therefore Christ is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, now that He has died to redeem them from the transgressions committed under the first covenant.