1)
What is the tense of "Hishameru... Al Tivtechu"?
1.
Metzudas David: They are careful, and do not trust one another.
2.
Malbim: This is a command - be careful, and do not trust one another.
2)
Why does it say "Kol Ach"?
1.
Malbim: Even a full brother from the same father and mother. Even though he will not actively harm his brother, do not rely on him to help. From Re'a (a friend), one must guard lest he harm him.
3)
What is the meaning of "Akov Yakov"?
1.
Rashi: They wait in ambush.
2.
Radak: This refers to scheming, like "va'Yakveni Zeh Fa'amayim" (Bereishis 27:36).