1)
Why does it say "v'Amarta"?
1.
Radak: Also say the curses to them, like written in the Torah in Sinai (Bechukosai) and Arvos Mo'av (in Ki Savo). Perhaps they will fear, and repent.
2)
Why does it say "Arur ha'Ish"?
1.
Malbim: He is cursed by himself. Due to abandoning Divrei ha'Bris, Hashem abandons him.
