1)
Why did he say "va'Ani ch'Cheresh"?
1.
Rashi: Yisrael hear their shame from those who revile them, and Yisrael do not answer, for we hope to You (verse 16).
2.
Radak: I hear their words via others, or from them, for they do not hide them from me. I act like a Cheresh who does not hear.
3.
Malbim: When they plotted to kill me, I pretended not to hear. They spoke in front of me, and thought that I do not hear.
2)
Why did he say "uch'Ilem Lo Yiftach Piv"?
1.
Radak: I hear my shame and do not answer [Malbim - to refute their words and clarify my virtue].
