Why did he add "u'Nekalosi Od mi'Zos"?
Rashi: [In front of Hashem,] I will belittle myself even more than this.
Malbim: He already told her that his honor does not depend on the nation, and rejoicing in front of Hashem glorifies himself - "those who honor Me, I will honor them" (1:2:30). If he lacks external signs of honor, he has internal, intellectual signs of honor. It would be even better if I did not want honor at all and fled from it. If I recognized Hashem's greatness and man's lowliness, I would be honored also in front of the slaves. True honor is the same in front of all people.
Why was David lowly in his own eyes?
Mesilas Yesharim (Perek 22 DH Im Ashir): If a person has wealth or strength, he should use it to help others. This is like a servant in a house; he must fulfill his assignment. There is no room for pride. If one investigates this with straight intellect, then he is truly humble, like David said about himself.
Why did he say "v'Im ha'Imahos Asher Amart
Radak (20, citing Midrash Shmuel 25:6): Heaven forbid, you are not called with the Imahos. (Also they put Hashem's honor above their own. - PF)
How will David be honored in front of the slaves?
Rashi: They do not disgrace me for what I did. I am honored in their eyes due to it! Radak - being lowly in front of Hashem is honor. Others will honor me for this, unlike you.
Malbim: Refer to 6:22:1:2.
