1)
Why does it say "Lema'an Hakim Es ha'Shevu'ah"?
1.
Malbim: The Shevu'ah to get Eretz Yisrael was on condition that they guard the Bris. If they do not guard it, they must be exiled from the land.
2)
Why did he say "Amen"?
1.
Rashi: [One who does not heed the Bris] will be cursed.
2.
Malbim: Yirmeyah accepted to guard the Bris, even if Yisrael will abandon it. The Shevu'ah to be to us Elokim, and to give to us the land, is not Batel as long as one of their descendants guards the Bris.
3)
Why did he say "[Amen] Hashem"?
1.
Radak: "Hashem" is Keri'ah (he calls to Him) and Tefilah (Hashem, so it should be).
