Not at all. There is no Bibical requirement that the Son must have a sinful nature in order to experience human weakness. Then he magically develops divine attribute of righteousness from human weaknesses. Like the Son's lack of knowledge is not a sin according to the human nature. The Son knows what it is like to be tempted, but he does not know what it is like to sin. And the Son’s inability to sin does not make his temptations less genuine. After all, the Son has experienced a real physical birth, to grow bodily, to increase in wisdom, to be accident prone, to have sufferings, to have infirmities, restrictions, and limitations by the normal constraints of his humanity without sin, even death. This is real in the experience of the Son as any other human being.
Not a method, but a Biblical doctrine called "Impeccability of Christ." Maybe if you drop the idea that the Son has a sinful nature. Because it is affecting the way you think and coming up with distorted views especially about the 'Imputed Righteousness." You also might be conflating "sin" with "weakness." Having bodily weakness is not a sin, temptation is not a sin, and even anger is not a sin. For example,
Was Christ sinning during the crucifixion?
2 Corinthians 13:4 For to be sure, he was crucified in weakness, yet he lives by God’s power. Likewise, we are weak in him, yet by God’s power we will live with him in our dealing with you.
Not at all. The Son's human weakness doesn't produce "righteousness," but rather it produces "obedience." The Son, as God in the flesh, is not going to zap his holy sinless human nature every time he encounters a weakness. Under this economic, he chose not to prevent but allows the human nature to function within its own respective laws. He economically submitted himself in the cooperation to his own flesh. Which is done for us and for our salvation to bring life and peace. Not to a fallen sinful nature that bring only sin and death. But rather, he accepts the weakness for what it is, conforms to it, experiences it, and becomes obedient to it in order to be our High Priest.
Hebrews 5:8-9 Son though he was, he learned obedience from what he suffered and, once made perfect, he became the source of eternal salvation for all who obey him
There is none that is righteous. And no man-made works can produce an equivalent righteousness on the same value level of God's righteousness. The Son according to the human nature doesn't develop holiness and righteousness. And then, once perfected that righteousness, it magically turn into a divine attribute equally to God's divine attribute of righteousness. Thus, having the character of God in fulness, which is absurd. Our imputed righteousness is not produced by a man and then given to us. As I have said many times before in this Trinity forum. If you really examine and scratch underneath the surface of Unitarian's Christology, then they turn Jesus Christ into some kind of demi-god. They won't admit to it, but that is exactly what they are doing in their Christology.
Philippians 3:9 and be found in him, not having a righteousness of my own that comes from the law, but that which is through faith in Christ—the righteousness that comes from God on the basis of faith.
Romans 3:21-22 But now apart from the law the righteousness of God has been made known, to which the Law and the Prophets testify. This righteousness is given through faith in Jesus Christ to all who believe. There is no difference between Jew and Gentile,
2 Corinthians 5:21 God made him who had no sin to be sin for us, SO THAT IN HIM WE MIGHT BECOME THE RIGHTEOUSNESS OF GOD.
The Hypostatic Union teaches there is only "One Person of Christ." We don't believe that "God the Son being injected into Jesus," which is two-persons and Nestorianism. And we don't believe the Son's human nature is a fallen and sinful. Which brings sin and death. We believe the Son-Person is impeccable or not capable of sinning. And his human nature is holy and not affected by original sin. Even during his temptations there was no sinful fleshly desires. He experienced temptation (which is not a sin) but never experienced that desire of a sinful nature of being "dragged away by their own evil desire and enticed" because desire produce sin and sin produce death. It's impossible to have a sinful nature and not commit sin. You will fall short of the glory of God.
James 1:14-15 but each person is tempted when they are dragged away by their own evil desire and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.
The Son has no sinful nature and sin living in him:
Romans 7:15-20 I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. For I know that good itself does not dwell in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.
The Son has no fleshly desires in conflict with the Holy Spirit:
Galatians 5:17 For the flesh desires what is contrary to the Spirit, and the Spirit what is contrary to the flesh. They are in conflict with each other, so that you are not to do whatever you want.
The Son having a fallen sinful nature is pure fiction and unbiblical. Therefore, the MESSIAH is the Jehovah tsidkenu "The Lord Our Righteous Savior" (Jeremiah 23:5-6) and "the Righteousness Of Our God and Savior (2 Peter 1:1).