As for asking who is David's father, it could be because Saul promised to reward the man who killed Goliath by giving his father’s house exemption from taxes in Israel.
1 Samuel 17:25 Now the Israelites had been saying, "Do you see how this man keeps coming out? He comes out to defy Israel. The king will give great wealth to the man who kills him. He will also give him his daughter in marriage and will exempt his family from taxes in Israel."
As for Saul commenting on David's appearance, it could very well be to draw attention to the amazement of being such a young person yet such a fierce warrior.
David was a young man. Maybe when he played the harp he was just a teenager, and then later grew up to be a fighter with facial hair and muscles. I can hardly believe you want to cast doubt on the whole Bible because of some things you do not understand.
Or you are making excuses for the editing done by the people who killed Jesus. Its obvious outside of the bias. Only God is perfect.