Why would he go out to the field?
Malbim: To be saved from the hunger - 'when there is hunger in the city, spread your legs' (Bava Kama 60b, i.e. leave the city).
Why does it mention those sick from hunger, but not those who died from hunger?
Radak: There were more than sick from hunger than those who died from hunger.
Who are "Gam Navi Gam Kohen"?
Radak: They are the false Nevi'im, and the Kohanim from Levi (Bnei Aharon). Some of them were evil - "also your brothers and father's house, also they betrayed you" (12:6). It says "Gam", for also they, the greats of the nation, were exiled with Yehoyachin, ha'Charash veha'Masger.
Radak citing Targum Yonasan: Also teachers [and Kohanim].
What is "Sacharu El Eretz v'Lo Yada'u"?
Rashi: They went around seeking salvation to a land that they did not know until now.
Radak: They went around to go to exile to another land. They did not know or feel themselves [that this will happened] until they were exiled. Even though [Yirmeyah] prophesized about the Nevi'im that they will die via sword and hunger, some were exiled with Yehoyachin, like it is written that they prophesized in Galus. It says Sacharu, for one who goes a long way, the path must turn; it cannot be straight all the time.
Malbim: They made Sechorah (business) to the land, like a merchant who sells merchandise in order to profit, so they sold Torah and Nevu'ah for money, and did not know what will be their end. This is a severe illness, for there is no one to heal it, i.e. teach them and bring them to repent!
