1)
Why did he call the Kohanim?
1.
Ibn Ezra: They make people swear - "v'Hishbi'a ha'Kohen Es ha'Ishah."
2.
Malbim: An oath via Kohanim is more stringent - "v'Hishbi'a ha'Kohen Es ha'Ishah bi'Shvu'as ha'Alah."
