"So, people are born with defects of character (sins of ignorance} which they will later manifest and develop."
1) Will they manifest and develop all of them and become guilty of them?
2) Or, will they avoid manifesting and developing some of them?
3) If so, are they guiltless of the ones they do not manifest and develop?
4) Why do sins of ignorance require a Savior if people are guiltless?

Yes, I believe children begin sinning shortly before they are born.

M: So, your answer to my question is - Paul sinned until the day he died.

R: No! You asked: "Did Paul ever reach the point where he was no longer practicing the sinful habits he cultivated while in the world?" My answer was that yes, Paul reached the point where he was no longer practicing the sinful habits he cultivated while in the world, otherwise he wouldn't have said, "Be imitators of me, as I am of Christ" (1 Cor. 11:1). This doesn't mean that, at this point, he was totally exempt from defects of character. We are born with natural defects of character which we will have to subdue until our vile body is changed.

"Christ will never neglect the work that has been placed in His hands. He will inspire the resolute disciple with a sense of the perversity, the sin-stained condition, the depravity, of the heart upon which He is working. The true penitent learns the uselessness of self-importance. Looking to Jesus, comparing his own defective character with the Saviour's perfect character, he says only-- 'In my hand no price I bring; Simply to Thy cross I cling.'" {HP 64.6}

This insight does not mean they are guilty of sinning..,

Through the power that Jesus gives, we can be "more than conquerors." But we cannot manufacture this power. Only through the Spirit of God can we receive it. {HP 64.2}

We need a deep insight into the nature of Christ and into the mystery of His love, "which passeth knowledge" (Ephesians 3:19). We are to live in the warm, genial rays of the Sun of Righteousness. Nothing but Christ's loving compassion, His divine grace, His almighty power, can enable us to baffle the relentless foe and subdue the opposition of our own hearts. What is our strength? The joy of the Lord. Let the love of Christ fill our hearts, and then we shall be prepared to receive the power that He has for us. {HP 64.3}

Let us thank God every day for the blessings that are ours. If the human agent will humble himself before God, . . realizing his utter inability to do the work that needs to be done in order that his soul may be purified; if he will cast away his own righteousness, Christ will abide in his heart. He will put His hand to the work of creating him anew, and will continue the work till he is complete in Him. . . . {HP 64.4}

Beholding Christ for the purpose of becoming like Him, the seeker after truth sees the perfection of the principles of God's law, and he becomes dissatisfied with everything but perfection. . . . But he knows that with the Redeemer there is saving power that will gain for him the victory in the conflict. The Saviour will strengthen and help him as he comes pleading for grace and efficiency. {HP 64.5}

Christ will never neglect the work that has been placed in His hands. He will inspire the resolute disciple with a sense of the perversity, the sin-stained condition, the depravity, of the heart upon which He is working. The true penitent learns the uselessness of self-importance. Looking to Jesus, comparing his own defective character with the Saviour's perfect character, he says only - "In my hand no price I bring; Simply to Thy cross I cling." {HP 64.6}