What Chesed did Nachash do with David?
Rashi, citing Tanchuma Vayera 25: When David fled from Sha'ul, his parents and brothers came to him, and he left them with Melech Mo'av" (1:22:4). Melech Mo'av killed all of them, except for one brother, who fled to Nachash in Eretz Amon, who kept him alive.
Why did David send only after Nachash died?
Malbim: Had he sent in his lifetime, he would have needed to make a Bris with the entire nation, which is forbidden.
It says "Lo Sidrosh Shelomam v'Tovasam" (Devarim 23:7). Was it proper to send to Chanun?
Radak citing Tanchuma Pinchas 3: No. He was punished; he sent to them with mercy, and came to disgrace (Chanun shaved half the beards of the Sheluchim and tore their garments - verse 4) and needed to fight against four nations (in this Perek). Hashem rebuked David - "Al Tehi Tzadik Harbeh" (Koheles 7:16)!
Malbim citing Hagahos Maimoniyos (Hilchos Melachim 6:6, citing Re'em): Yes. If an Amoni or Mo'avi did Chesed for you, you may repay Shalom. Malbim - this applies also to consoling him over the loss of his father. This honors his father; it is like returning Chesed. David was punished (refer to 10:2:4:1) because it looked like his Sheluchim went "Eretz Bnei Amon", to seek the Shalom of the entire nation.
Why does it say "El Aviv"?
Radak: This is like "Al Aviv". "El he'Harim Lo Achal" (Yechezkel 18:6) is like Al he'Harim.
Malbim: He sent for [the sake of] Chanun's father, and not for Chanun himself.
