"When has the line been crossed (if ever)?"
To turn from one’s own choice for himself, to God’s Choice for him, would be to repent. But what line are you talking about?
"1) What act is beyond repentance (if any)?"
To not repent.
"2) Is "to sin in the knowledge of sin "unforgivable"?"
No, what is unforgivable is to refuse the Presence of God. One can repent of sin even if he knows he is sinning.
"3) If God (biblical) only forgives those who forgive themselves - What is the point of God's forgiveness?"
“If God (biblical) only forgives those who forgive themselves “ This perception is erroneous to what the Bible says. God expects His to fogive because He being the Righteous Judgement in Mercy has forgiven their trespasses .
God’s forgiveness is to not hold the lack of the fulfillment of His Commandments against you. Even... Gen:2:17: But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
Adam could not fufill this commandment and live after he and his wife ate of the tree. And Jesus being the Word of God in the Son of Man who did not eat of the tree, was able to complete fulifill this commandment as well as all the rest, and live.
Therefore in Him, meaning Jesus is the forgiveness of God to man. Making Peace betwixt Him and man.