Given that the story of Adam and Eve is mythology and that, while there obviously had to be "first" man and a "first" woman, it does not follow that both evolved at the same time and the same place nor that they are humanity's "first" parents.  The theology of Original Sin is without foundation, ergo, so too is the Immaculate Conception.  If we accept that God is both creator and ultimate author of Scripture, then our theology must never fail to respect the authority of both science and scripture/tradition if it is to be viable.
Furthermore, how valid is it to suggest that anyone can be guilty because of another's sin.  Yes, I can cause another to sin, but I will never believe my personal guilt can be passed on to another.