1)
What do we learn from "v'Shoketes u'Shlevah"?
1.
'Rashi': They used to dwell serenely. No one ever did evil to them. Therefore, they were not careful when [Bnei Shimon] came to fight them. This is like Layish - "Al Am Shaket u'Vote'ach? u'Davar Ein Lahem Im Adam." Because they were serene, they did not fear, and they did not have a Bris with anyone to help them. Therefore, 500 [Bnei Shimon] defeated them. It says there also "k'Mishpat Tzidonim Shoket u'Vote'ach."
2.
Malbim: They did not have external war, nor internal rebellion. Therefore, they were not prepared for war.
2)
Why does it say "Ki Min Cham ha'Yoshevim Lefanim"?
1.
Malbim: They were established in the land from a long time ago.
