Why is "Gam Evvil Macharish Chacham Yechashev"?
R. Yonah: This is counsel for an Evvil, to be silent and hear. He will not be considered lacking Binah, rather, a Chacham. People will say that he has Chachmah, for a Chacham normally listens, is quiet and does not reveal his opinion - "v'Ozen Chachamim Tevakesh Da'as" (18:15). Via this Midah, he will learn Chachmah and gather its grain.
Malbim: Even an Evvil, who is unsure about everything, if he is silent and does not ask this (why Tzadik v'Ra Lo), he is considered a Chacham, for he conducted according to the laws of Chachmah, not to say such questions, even if he is unsure in his heart.
Why does it say "Otem Sefasav Navon"?
R. Yonah: The Midah of Navon is when he wants to speak and tell his opinion, for "Simchah la'Ish b'Ma'aneh Fiv" (15:23), he closes his lips and rules over his Ru'ach from answering, until after deliberation and thought.
Malbim: If [an Evvil] totally seals his lips, and even in his external speech is not found any doubt about such matters (Tzadik v'Ra Lo), he is considered Navon. People will think that he reached the level to contemplate with Binah - "Yekar Ru'ach Ish Tevunah" (27).
