1)
Why did he say "Ein Kamocha Elokim"?
1.
Malbim: Hashem made two promises to David. (a) Shlomo will rule after him and build the Beis ha'Mikdash. This was unconditional. (b) Also his descendants will sit in the throne. This was on condition "Im Yishmeru Vanecha Brisi v'Edosi
2)
What is the meaning of "Shomer ha'Bris"?
1.
Rashi: He waits and is Gonez 1 to guard the Bris and Chesed, to fulfill His promise.
2.
Radak: He guards the Bris with the Avos.
1
This is a metaphor, as if He buries it to store it for future use. (PF)
3)
To whom does "la'Avadecha" refer?
1.
Radak: It is the descendants of the Avos.
