I never really put much thought into the "validity" of original sin.
To be truthful that thought of someone dying via torture to atone for the mistake that two people made 1000's of years before so that current and future generations have a chance at salvation, always disturbed me.
God knows US, he doesn't need an act by his Son or by US to KNOW that we have repented or that the mistake of others has been atoned for, that would make him NOT omniscient.
So what was the ransom for? why did Jesus die horribly on that cross?
Paul says that only the (willing) sacrifice of a perfect human (Jesus) could the mistake of the (then) perfect Humans be atoned for.
Adam and Eve's fall from Grace was atoned for by Jesus's willingness to Die for Us, ie: we deserve God's grace so we have it back.
That wich was lost has now been given back to Us.
Or something like that...