1)
Why should Hashem pardon him?
1.
Malbim: (a) Due to a gift of undeserved Chesed. (b) Amidst mercy, when He sees the sinner's crushed heart and submissiveness.
2)
Why did he say "k'Chasdecha"?
1.
Radak: It should not be according to my deeds.
3)
Why did he say "k'Rov Rachamecha"?
1.
Malbim: This is a second reason (refer to 51:3:2:2) why Hashem should pardon him - amidst mercy, when He sees the sinner's crushed heart and submissiveness.
4)
Why did he say "Pesha'ai" (plural)?
1.
Radak #1: For this sin, and others.
2.
Radak #2: There were two aspects of this sin - Bas Sheva, and Uriyah.