1)
Verse 14 already said "va'Ani ch'Cheresh"!
1.
Radak: Here it means that I do not understand the language that they speak. This is like "Goy Asher Lo Sishma Leshono" (Devarim 28:49).
2.
Malbim: Really, I heard everything, just I acted like a deaf-mute and one who has no reply.
2)
Why did he say "v'Ein b'Fiv Tochachos"?
1.
Radak: I am like one who has no claims to argue with those who quarrel with him.
