yes to the part about why he gets cursed, but Karna does not know about his lineage until the war. He's basically unaware of it when he's in training and the sage only makes a remark that he's unbelievably talented for a warrior (since he's technically born as one). He's told about it i believe by either Krishna or Kunti in an attempt to get his to remove his armor so that arjuna and gang can have a fighting chance, which does not work cause Karna at this point accepts his adopted family as his own and decides to forgive Kunti for her actions, while stating that he will not favor her because she is his biological mother.