1)
What is the meaning of "v'Lo Yelech Na Itanu Amnon"?
1.
Rashi: If you (David) cannot come, Amnon should come with us. Malbim - since he is the Bechor, he is in place of the king.
2.
Radak: "V'Lo" introduces a request, like "v'Lo Yutan l'Avdecha" (Melachim II 5:17).