1) Imputation, by definition, is not "actual," it is reckoned, accounted, ascribed as though it were actual.
As in adoption does not make one an actual biological child, it makes one (legally) counted/reckoned/ascribed as though a biological child.
Abraham was imputed/reckoned/accounted as righteous (Ge 15:5, Ro 4:1-5), he was not actually righteous (sinless), he was still sinful with a fallen nature.
2) Adam's fallen (sinful) nature was actually caused by his fall (disobedience) which we now inherit as his descendants, which fallen nature is incapable of perfect obedience, and which disobedience/sin is the cause of man's condemnation at the judgment.
In addition to being guilty of sin committed due to his fallen human nature, man is also guilty of Adam's sin imputed to him (Ro 5:14 17-19),
which is why all died between Adam and Moses ("sin was in the world") when there was no law to sin against and, therefore, they were not guilty of any personal sin (Ro 5:12-14) to cause their death.
They were guilty of Adam's sin imputed/reckoned/accounted to them (Ro 5:17), which imputation of Adam's sin to them is the pattern of the imputation of Christ's righteousness (Ro 5:14. 18-19).
3) Imputed righteousness does not make one actually (by action) righteous,
one is only reckoned/accounted/ascribed as righteous, and outcomes of both the actual and the imputed are the same--children of God.
Imputed sin of Adam does not make one actually (by action) guilty of Adam's sin, only reckoned/accounted/ascribed as guilty, the outcomes of both the actual and the imputed being the same--eternal condemnation.
Fallen nature and imputation of Adam's sin are two different things.
Fallen nature is inherited, it is actual, it is not imputed (accounted, reckoned).
The guilt of Adam's sin is imputed (accounted, reckoned), it is not inherited, it is not actual for man does not inherit the guilt of his father's sin.
All mankind is condemned by the sin of Adam imputed to them (Ro 5:12-14) without ever committing an act of their own.
Imputation, by definition, is not actual.
Righteousness can be imputed, righteousness can be actual, righteousness can be non-existent, etc.
There are two kinds of righteousness:
justification - sentence of acquittal, declared "not guilty," sinless, forensic righteousness (imputed because one is now sinless)
sanctification - actual righteousness of obedience in the Holy Spirit.
The "righteousness of Christ" imputed to us in justification refers to the forensic righteousness imputed to us due to the death of the righteous Christ which removed our sin.
Our actual righteousness is by sanctification through obedience in the Holy Spirit.
I suspect this is all as clear as mud.