The expression "last Adam" is found in Paul's discussion of the resurrection found at 1 Corinthians 15 (verse 45). It is also based on Paul's comparison found at Romans 5, "For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous." (verse 19)
There is no doubt that atonement for human sins is taught in the Bible to accrue from Christ's death, and that this death is in some sense a sacrifice and a ransom. But a detailed theory of atonement is not worked out in the Bible. The WT idea of the "ransom," current ever since the time of Russell, is one form of the substitionary theory of the atonement - that Jesus died as the sinner's substitute. According to the WT, he was actually the substitute for Adam (all mankind being in Adam's "loins" - or in his genes as we would now say). Just as Adam forfeited a perfect human life through disobedience, so Jesus sacrificed a perfect human life to make up for Adam's transgression.
The perfection referred to includes sinlessness, with the potential of living forever free from old age and sickness. This is the only equivalency that is required. The equivalency is similar to that mentioned in the U.S. Declaration of Independence - "All men are created equal ..." They are equal as human beings before the law, with certain inalienable rights. Nothing is said about their intelligence, their education, their natural abilities, and so on. Just so, Jesus and Adam were equal as PERFECT human creatures.
The fact that Jesus kept his integrity under trying circumstances that Adam did not face, as foreshadowed by Job, is another matter. The WT, to my knowledge, has never claimed that Satan was able to boast over his conquest of Adam and Eve as he could have done if Job had failed - to Satan, Adam and Eve were a piece of cake. It is to the WT's discredit that it does not accept the idea that Adam and Eve will not ultimately be held responsible for their error but will instead themselves come under the benefits of the ransom. This, in fact, is the view that Pastor Russell held and which was later changed by Rutherford. But even Job did not keep his integrity perfectly - as the WT has shown, the dialogues between Job and his three friends reveal some self-righteousness on Job's part. Job kept his integrity as an imperfect man, whereas Jesus kept his as a perfect man.
What ultimately gave Jesus' human life its ransoming value, though, was not its equality with the life of Adam. Jesus was the incarnation of the Second Person of the Blessed Trinity, and as such was himself God. The union of divinity with a perfect human life gave that life infinite value, and the One who was sacrificed was not a mere creature who was entitled to his own life rights which could not justly be substituted for those of others, but the Creator himself.
Justin