Was the sin of man and the death, that sin brought forth, absolutely necessary for the destruction of the devil and his works?
No.
From Hebrews 2:14 that through death he might destroy him that had the power of death, that is, the devil;
Was that not going to be the case, the truth, even before the foundation of the world?
Well, it's a bit bigger than that. This Creation was not about destroying the Devil and His Works. It was about glorifying and exalting the Son, and showing the riches of His love and goodness to those who would be the heirs of salvation.
God is also going to destroy the Devil and his works, but He is not constrained by any necessity to create man and die for man in order to do so.
Otherwise, why would God foreordain, before the foundation of the world, that the sinless Christ would shed his precious blood, because of that sin and death?
Again, to glorify and exalt His Son, and to demonstrate His love and goodness and His power to those who would be heirs of salvation.
If Adam had not sinned, would the obedience of one, by which many would be made righteous, have come through Adam?
Was that the plan of God but Adam fell, relative to that plan?
No. Salvation could only be wrought by One who is incorruptible and Who had the power of an endless life. Adam was good, but was corruptible and mortal. He was tempted of the Devil, and He was corrupted and died, and death holds him. Jesus could not be corrupted. He was tempted of the Devil, and was not corrupted (indeed, could not be, because He is God [and Adam is not]) and though He was given a flesh and blood body so that He could be weak enough to taste death, death could not hold him, and He was raised and made a High Priest by the power of an endless life.
I believe the creation, that which was stated as being very good, and included Adam, was subjected to futility, Rom 8:20 for a purpose.
Yes, and that purpose is the glory and exaltation of the Son.