1)
Why did they think that they are spies?
1.
Rashi: [Jews] are commanded "Lo Sidrosh Shelomam" (Devarim 23:7)!
2.
Malbim: They did not come like those who come to make a Bris with the living king, like Chiram sent to David and Shlomo.
2)
What is the meaning of "u'Lehafchah"?
1.
Radak citing Targum Yonasan: It is to examine. One who examines something, he turns it from side to side.
2.
Malbim: To [conquer it and] convert it [to become part of Yisrael].
