1)
What are "Mesei Shav"?
1.
Ramban: They are liars.
2)
Why does it say "v'Lo Yisbonan"?
1.
Rashi: Hashem acts as if He does not understand. He delays His anger, and lets them live many years.
2.
Ibn Ezra: He sees the sin, and need not contemplate.
3.
Malbim: [Who can say] 'You act as if You did not know that he will sin, i.e. he had choice'? Really, even though 'ha'Kol Tzafuy (everything is seen, He knows the future), ha'Reshus Nesunah (man has choice; therefore), b'Tov ha'Olam Nidon' (the world is judged properly - Avos 3:15).
