David, I'm not sure you understand Jacob's question. Jacob is asking, "if we all sinned in Adam geminally (that is to say, in the loins of Adam, just as Levi paid tithes to Melchizedek in the loins of Abraham, from whence the author of Hebrews proves the supremacy of Christ's priestly office, then the imputation of Adam's sin as our federal head seems redundant. So why then do we need Adam to be our covenantal head if we sinned in a realist way in Adam?"
I think it's an excellent question. Not sure as to the solution. I find trudicianism more plausible in many regards. However, Rom. 5 is difficult. There seems to be an isomorphic structure to Paul's argument such that it is exegetically forced to assume imputation of Christ's righteousness on one hand, and a realistic view with respect to Adam's sin.
Nonetheless, I still lean towards traducianism, at least with respect to the origin of the soul.