1)
Why did he tell David (but not his men) to return?
1.
Malbim: He said that David could go to fight with the soldiers - but Achish does not want to lower his honor. Therefore, David should return, and his men will fight.
2)
What is the concern if Sarnei Pelishtim will be upset?
1.
Malbim: They will fight you.
