A quick answer to the question of how Christ is the 2nd man, when He wasn't the 2nd man in the natural order, is that God only counts the redeemed. He only counts those who are alive unto Him. And all of the redeemed can only be counted in Christ and from Christ. Christ cannot be the first Man of a new race. He must be the 2nd Man of a fallen race in order to be able to redeem that race.
So Christ is the Last Adam, the last representative of Adams race. He is the 2nd Man of that race, in that only the redeemed are counted. And God counts all the redeemed in Christ. Or as 1Cor. 15:22 says, " For as in Adam all die, even so in Christ shall all be made alive." Because all are redeemed in Christ, then the count of the redeemed begins with Christ, as the 2nd Man. In other words, because He is called the 2nd, then there will be a third and fourth, etc. etc. you, me, etc. etc.
So, all of Adams race is completly identified as dead and gone in him. So much so that Christ is called the 2nd Man. Christ is another and last Adam in that there will be no more. He is the 2nd Man in that there will be more to follow.
Other things come into play which I am trying to understand, such as Eve the mother of all living and the Seed promises. But this is my rough explanation.
Quantrill