Even the Catholics don't hold that original sin is, in fact, sin in the literal sense that you and I, along with everyone else, is guilty of an actual sin. Rather, we carry the burden of the punishment that God placed on mankind; that is, we live in a world where we have to toil for the fruits of our labor, and we aren't immune from suffering.

As for Leviticus and the commandments in the Bible prior to the New Testament, that has to do with the covenant that God had with Moses, which was fulfilled by Christ's sacrifice on the cross. He made a new covenant with us through his Son's sacrifice, leaving our salvation in our own hands and binding us not to the enumerated laws of the halakhah, but to the Noachide laws (essentially, the first ten Commandments that were renewed in the covenant he made with Moses on Mt. Sinai).

As for why people pick and choose which passages they choose to follow? That's up to the individual. There are a myriad of reasons for the cognitive dissonance that plagues some Christians as to saying they follow the Lord and not following the Lord in their deeds. I'm guilty of not living up to the covenant that Jesus made with mankind, but I at least admit to it and strive to be better. In the end, we're all imperfect beings.

That's just my take on it and my attempt to answer your questions. I welcome your reply.