Culpability and original sin

So part of the standard story that you get in Christian theology is that everyone is sinful and requires redemption from God.  But why is everyone sinful?

Here are a few ways to answer this question.

Everyone is already sinful at conception, birth, or whenever you have the emergence of a human person.

Everyone will eventually sin at some point in their lives, when they do, they are sinful and need God's redemption.

Let's start with that second answer.  Is it true that EVERYONE will eventually sin?  Is it possible for someone to go their whole life without actually sinning?  It seems that the general consensus answer among theologians is no, although I am not entirely sure.  If the answer is no, then the next question is, "why not?"

One answer is that everyone is born with a defect that will inevitably result in the committing of a sin of some sort.  Okay, so where did this defect come from?

Now we have two choices, either people are sinful, i.e. guilty before God, right from the start, or they have a defect that will inevitably lead to sin.  Either option raises the same basic question: why?

The standard answer is that we inherited this condition from Adam, i.e. the first man and also the first person (along with Eve) to sin against God.  Because of Adam's sin, we are also sinful.  This is puzzling.  Why should I, or anyone else, be held responsible for the acts of one man (and woman)?

Let's suppose that it is indeed the case that I am sinful before God because of the disobedience of Adam.  it seems to follow that I am in some responsible for what he did.  If that is true, then shouldn't it also be true that I am responsible for the acts of my parents, grandparents, and all of my ancestors?  If not, why not?  Why is the case with Adam exceptional?  How is it that the sin of Adam is transmitted through the generations, but no one else's?