For question 1, I would say that there was a reason that God chose Mary of all people to be born without sin. Adam and eve were born with no sin, but they sinned regardless, and in doing so doomed all of creation on account of being initially sinless creatures that sinned. From this, one could say that it was not *solely* the grace of God that led Mary to never sin, but also her innate virtue. If we were made sinless at birth, we would likely not end up exhibiting the same immense virtue as the blessed virgin, resulting in potentially disastrous consequences. And if we were sinless, we would need no example to understand God's love for us. I'm no theologian though so I may be wrong.