Why did he say "Gam Hashem He'evir"?
Radak: Just like you confessed, Hashem accepted your Viduy and Teshuvah.
Why did he say "Lo Samus"?
Radak: Even though you are Chayav Misah, Hashem accepted your Teshuvah, and you will not die. I.e. you will not die like Resha'im, that the Nefesh descends to Gehinom. Rather, you will be punished in this world. The punishment for the Bi'ah is "v'Shachav Im Nashecha", and includes Avshalom's rebellion. He could not lie with your wives without rebelling! For killing, "the sword will not depart from your house" (verse 10), and the son who will be born to you, who was conceived in sin, will die.
Why did David keep his kingship in spite of his sin? Sha'ul lost kingship due to one sin!
