1)
Why does it say "Ad Henah Divrei Yirmeyah"? He said also Perek 52!
1.
Radak #1: It is because he elaborated about the evil that will come upon Bavel. It similarly says above (48:47) "Ad Henah Mishpat Mo'av."
2.
Radak #2: From here until the end of the Sefer recounts events. It is not Yirmeyah's Nevu'ah. Rather, the one who wrote the Sefer wrote at the end the exile and the Churban, like it is written at the end of Sefer Melachim.
3.
Radak, based on the Rambam: Nevu'ah must leave a Navi before he dies, either a long or short time. When Yirmeyah finished his Nevu'ah about Bavel, Nevu'ah ceased from him.
