1)
Who are "Peletim mi'Cherev"?
1.
Rashi: They are Yisrael exiled to Bavel; they escaped the sword of Nevuzaradan.
2)
What is the meaning of "Zichru me'Rachok Es Hashem"?
1.
Radak, Targum Yonasan: Remember from a long time ago - 70 years, the Avodah of Hashem that was removed.
2.
Malbim: Remember Him, to return to Him.
3)
How should they put Yerushalayim on their hearts?
