1)
What does this add to the previous warning?
1.
Radak: Above (verse 2), Hashem commanded him "El Ish Yehudah [v'Al Yoshvei Yerushalayim]". Here he explains that he should not wait for them to come to Yerushalayim. Rather, he should go to Arei Yehudah and say all these words to them. Perhaps they will repent due to hearing the curse and warning. Also in Yerushalayim, he should speak b'Chutzos - in public, in front of the entire Am.
2.
Malbim: Now He made him a Shali'ach to say so b'Chutzos Yerushalayim, in public, like one who warns about a punishment destined to come.
