Why does it say "b'Achas (feminine) ha'Pechasim"?
Radak: Here the word is feminine; below (18:17 - "ha'Pachas ha'Gadol") it is masculine. 1
We find words like this (refer to Bereishis 32:9:1:1).
What does "ha'Mekomos" refer to?
Radak: They are places proper to hide.
Why did he say "v'Shama ha'Shome'a"?
Radak: At night, people cannot distinguish one camp from another. When David's camp kill some of our men, and they are intact, they will scream among them. Our men will realize that they are Giborim fighting fiercely and risking their lives. One [among our camp] who hears will think that David's camp were victorious, for people cannot tell at night!
Malbim: Not only will Achitofel's counsel not help -it will hurt! Even though only 12,000 go out the first time, people who hear [that they did not succeed] will not know the details, and they will say that Avshalom's camp fought and fell. This will melt the heart of the nation, who still fear David and his Giborim!