I am not quite sure how Jesus can die for future sins.
From the standpoint of most current mainstream Christian confessions (cf. your thread title) I see no reason why he could not.
From the NT perspective, the idea that Jesus dies for our sins (whatever the metaphor used to explain how that is supposed to work -- ransom, sacrifice of atonement, sacrifice of communion etc.) is widespread but not necessarily common to all segments of early Christianity -- for instance, it doesn't seem to play any role in Matthew (in spite of 20:28 which is directly dependent on Mark 10:45, and may well be understood in a different way) or the epistle of James, which suggests that early Jewish Christians circles did not really consider Jesus' death as the cause for forgiveness and salvation from sin and death (cf. also the non-sacrificial version of the Eucharist in the Didachè).
Where this idea became central it was most often linked to short-term eschatology. Jesus was understood to have appeared at the "end," not at the "middle" of history. But as time went by the perspective changed: every generation of Christians was concerned to have their sins (naturally committed after Jesus' death) forgiven, implying that the sins Jesus had died for were increasingly future sins. One widespread assumption was that baptism, realising the sacramental union with Jesus' death, cancelled previous sins -- but sins committed after baptism remained a big problem, leading to a frequent practice of deathbed baptism (giving little time for further sin) in later centuries. Of course the practice of confession and absolution became increasingly important as a result.
I think the deepest NT development on the issue is found in Johannism: Jesus takes away the sins of the world, beyond the group of belivers (John 1:29; 1 John 2:2). But people can "remain" and "die" "in their sins" (8:21,24; 9:41; 15:22ff; 16:8ff), or "sin no more" (5:14; cf. 8:11); the believers have the authority to forgive or not to forgive sins (20:23; cf. 1 John 5:14ff). A dualism emerges (not unsimilar to the later Lutheran doctrine of simul peccator et justus): believers are both sinners, continuously forgiven and cleansed (1 John 1:7--2:2,12), and absolutely sinless as God's children (3:4-9; 5:18)
Does this mean non Adamic sin? If so, does this suggest that in the future a perfect creature can sin and be forgiven of their sin, rather than be instantly destroyed as the Watchtower teaches?
As far as humans are concerned, I don't see any distinction between "Adamic" and "non-Adamic" sin in the NT. And the "world to come" is hardly described in temporal terms allowing for sin. This will be ruled out in later developments because of the philosophical notion of eternity as timeless rather than endless time, a notion shared by most church Fathers including Augustine who develops the notion of "original sin".
