1)
Who was the Kushi?
1.
Radak #1: He descended from Kush, and converted.
2.
Radak #2: He was a Yisrael as black a Kushi (Ethiopian), therefore he was called Kushi.
2)
Achima'atz had said "Arutzah Na va'Avasrah" (verse 19). Why did Yo'av say "Lech Haged"?
1.
Malbim: This is not good news, but in any case he must be told.
3)
Why did he say "Asher Ra'isa"?
1.
Malbim citing Pirkei d'R. Eliezer 52: He is the one who told Yo'av that he saw Avshalom hanging from the tree. You saw that it was Hashgachas Hashem that he be killed!