1)
What sign does he request?
1.
Rashi: It will be recognized to others that You pardoned me 1 . Hashem did not do so in his lifetime, only in the days of his son Shlomo [when he wanted to enter the Aron to the Mikdash]. The gates stuck to each other, and did not open until he said "Al Tashev Pnei Meshichecha Zachrah l'Chasdei David Avdecha" (Divrei ha'Yamim II, 6:42).
2.
Radak: It is the kingship. It will be a sign to my haters that You chose me from then.
1
Malbim: This corresponds to "Erech Apayim" (verse 15).
