1)
Why does it say "Ki"?
1.
Malbim: I do not trust in the fortress - You are my Rock and Fortress!
2)
Why did he say "u'Lema'an Shimcha"?
1.
Radak: Until now, You were my Rock and Fortress. Also from now and onwards, lead me for Your sake, and not of mine.
2.
Malbim: Since Your name is called on me - You are Ma'uzi - You will lead me [and take me out of the net that they set for me].
3)
What is the meaning of "Tancheni"?
1.
Rashi: This is [lead me,] like "Lech Neche Es ha'Am" (Shemos 32:34).