1)
Why did he say here that Hashem helps him?
1.
Radak: They cannot harm him via their Rechilus.
2.
Malbim: He helps David himself.
2)
Why does it say here Shem Adnus (written with Aleph-Dalet)?
1.
Radak: He is Master over everything; He will help me.
3)
What is the meaning of "b'Somechei Nafshi"?
1.
Radak: He will help those in Yisrael who support my Nefesh.
2.
Malbim: He will join to be among David's men and those who support him.