Why did they ask what Asham they should return?
Rashi: They were told (verse 3) "Tashivu Lo Asham", which implies the same matter that they sinned with. They asked how this can be.
Is there another source that there were five Pelishti nobles?
Rashi: Yes - they are listed in Yehoshua 13:3.
Why did they send gold Techorim and mice?
Radak: This supports the Midrash that they were afflicted by mice (refer to 5:6:3:1). Giving these for atonement would remove those afflictions.
Malbim: Presumably, the affliction took the form of the sin. Therefore, to return the Asham, the gift should resemble the affliction. Now you will show honor in place of rebellion, and return it towards Kedushah. The Techorim 1 are big, one to atone for each noble. The mice are small, one to atone for the people under each noble. According to the opinion that the mice destroyed the land's harvests, the mice 2 corresponded to the affliction of their produce.
Why does it say "l'Chulam"? It should say l'Chulchem!
Radak: We find like this - "Shim'u Amim Kulam" (Divrei ha'Yamim 2 18:27).
