Why did Yo'av say "no"?
Rashi (citing Sanhedrin 48b): Yo'av told Shlomo 'I will not accept a second punishment. David already cursed me for killing Avner 1 (Shmuel II, 3:29). If Shlomo will accept the curses on himself, I will leave [the Mizbe'ach and let you kill me].' Radak citing Sanhedrin 48b - Rechav'am was a Zav. It says "Rechav'am His'ametz 2 La'alos ba'Merkavah (12:8)", just like "ha'Merkav Asher Yirkav Alav ha'Zav Yitma" (Vayikra 15:9). Uziyahu was a Metzora - "veha'Tzara'as Zarchah v'Mitzcho" (Divrei ha'Yamim II, 26:19) Asa needed a cane - "l'Es Ziknaso Chalah Es Raglav" (15:21) Yoshiyahu was killed by the sword - "va'Yoru ha'Yorim la'Melech Yoshiyahu" (Divrei ha'Yamim II, 35:23) Yechonyah lacked bread - "va'Aruchaso Aruchas Tamid Nitnah Lo" (Melachim II, 25:30, but before this, he lacked food).
Refer to Shmuel II, 3:29:4:1*.
Rashi (48b): Merkavah is like Merkav; this is like an Asmachta for a Gezeirah Shavah. Radak - we learn from "His'ametz." It was hard for him to ride, due to his illness of Zivah. (Why did the Gemara cite the verse of Zav? Perhaps Radak means that we learn also from His'ametz. (PF)
Why did he say "Ko Diber Yo'av" and "v'Cho Anani"?
Refer to 2:30:1:1.
Radak #1: Verses often use two expressions for one matter. Yo'av said only "Lo Ki Fo Amus."
Radak #2, Malbim #1: Perhaps he said also other matters; the gist was "no, I will die here."
