Why did He say "Im Lo"?
Malbim: This is an expression of a Shevu'ah, like "Im Lo Batim Rabim l'Shamah Yihyu" (Yeshayah 5:9). The decree was with a Shevu'ah, so it had to be fulfilled.
Why is Achav liable for the blood of Navos' sons?
Radak citing Sanhedrin 48b #1: Achav killed also Navos's sons 1 .
Radak citing Sanhedrin 48b #2: The verse discusses sons who were proper to come from Navos. This is like "Demei Achicha Tzo'akim Elai Min ha'Adamah" (Bereishis 4:10).
Radak citing his father: "Demei Vanav" refers to stealing their inheritance from them.
Radak explained why Izevel needed a pretext to kill Navos; refer to Melachim I, 21:10:1:1. What was the pretext to kill his sons? (PF)
What is the meaning of "Ra'isi Emesh"?
Radak citing Targum Yonasan: It is revealed in front of Me, close.
Radak: Emesh literally means the night before. Hashem told the Nevu'ah to Eliyahu on the night after Navos was killed; Achav went to inherit the field the next day.
Did Hashem say that his body will be in this field?
Malbim: Refer to 9:25:4:1 and the note there.
