Who says so about Yirmeyah, and who says that they say so?
Rashi: Yirmeyah complains that Anshei Anasos say so.
Radak: Yirmeyah tells Hashem that the people he rebukes with Hashem's word, they say so.
Why do they say "Lechu"?
Radak: Let us be zealous to kill him via the tongue. This is like "Lechah Na Anasechah" (Koheles 2:1), "Lechu Na v'Nivachacha" (Yeshayah 1:18) and similar verses.
What thoughts do they want to have about him?
Radak: Evil thoughts, to kill him. Malbim - this is after he informed them that Hashem intends for their good 1 !
Right now, he just told them that they will constantly be fleeing in exile! Perhaps Malbim refer to verses 9-10; Hashem initially intended for their good, but retracted due to their deeds. (PF)
Why does it say "Torah mi'Kohen"?
Rashi: It is upon the Kohanim to teach Torah - "Yoru Mishpatecha l'Yakov" (Devarim 33:10).
Radak: "Lo Sovad Torah mi'Kohen", for even if he dies, there are other Kohanim who know the Torah. Also we have Nevi'im to tell us Nevu'ah, and unlike him - he prophesizes only bad for us!
Malbim: Yirmeyah had three attributes. (a) He was a Kohen who ruled about Halachah. (b) He was a Chacham to give good counsel. (c) He was a Navi who spoke Devar Hashem. We can kill him and find others with these attributes, albeit not all in one person. Other Kohanim have Torah (can rule). We will not lose "Etzah me'Chacham", and not "Devar me'Navi" (others remain).
How will they strike him with the tongue?
Radak: They will say bad about him to the king, so he will kill him. Malbim - this is after they tried to poison him (11:19), and failed.
Malbim: We will use his own words of Nevu'ah, and inform on him to the king. Even though there is merit for him in his final words - he prayed for the nation, and aroused them to Musar, that they should return to Hashem and Hashem will return to benefit them - we will not heed all his words, and tell only some, so he will be Chayav Misah.
What is "Kol Devarav"?
Radak: It is his many words.
Malbim #1: He will want to justify his words - we will not heed him.
Malbim #2: Refer to 18:18:5:2
