Your questions and observations seem quite contrary to the Protestant faith. I am not sure from what faith you are deriving these ideas. However I can say from the Protestant faith that Mary and Joseph were sinners and all are born into sin. Therefore Mary and Joseph did not need to posses the holiness of God; rather they needed the Christ to die for their sin:
Surely I was sinful at birth, sinful from the time my mother conceived me. (Psalm 51:5)
From Romans 3:9-18 we see how bad this state is:
9What shall we conclude then? Are we any better d?
Not at all! We have already made the charge that Jews
and Gentiles alike are all under sin. 10As it is written:
“There is no one righteous, not even one;
11 there is no one who understands, no one who seeks God.
12 All have turned away,
they have together become worthless;
there is no one who does good,
not even one.”
13 “Their throats are open graves;
their tongues practice deceit.”
“The poison of vipers is on their lips.”
14 “Their mouths are full of cursing and bitterness.”
15 “Their feet are swift to shed blood;
16 ruin and misery mark their ways,
17 and the way of peace they do not know.”
18 “There is no fear of God before their eyes.”
'All are under sin' includes Mary and Joseph, but not Jesus. The Protestant faith thinks that unless Jesus died for Mary, she would have been cast into hell upon her death. Catholics believe that Mary was immaculately conceived.
What was 'needed' was a virgin for the Spirit to create a body for the Christ, from her womb. God provided a body for the eternal Son to assume human nature through the Virgin Mary. This way Jesus was the only human not born in sin, because he was born a God-Man. He was God and Man in one person, human and divine. Mary was only needed so that the eternal Son, who is God, would become a human also. If Jesus has been born from Mary and Joseph, then he would have been a sinner needing a Christ to save him.
God needed to assume human nature so that He could offer the human soul and body of Jesus as an atoning sacrifice for the sins of the whole world, including the great and many sins of all those people with halos on paintings.
“For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. (John 3:16)