1)
Why did he say "Esmecha v'E'eltzah Vach"?
1.
Radak: I trusted in You when I went to fight [Galyus]; I am a youth, and he is a warrior.
2.
Malbim: My thanks to Hashem is unlike others'. Their primary Simchah is the good that they received; it is based on self-love. I rejoice in You, that via this Your great name is publicized. Via this (salvation) I can sing Your name and inform people that You are supreme and rule over everything.
2)
Why will he sing to Hashem's supreme name?
1.
Radak: You are elevated above everything; man's strength is Sheker.
