What is the meaning of "Tziyon Yish'alu"?
Radak, Malbim: When they return from Galus Bavel, they will ask what is the way to Tziyon.
What is "Derech Henah Feneihem"?
Rashi: This path goes to Yerushalayim; they will turn in that direction when Bavel is destroyed and they leave Bavel with permission from Koresh Melech Paras. Radak - "Henah" is this place, like "Lo Atem Shelachtem Osi Henah" (Bereishis 45:5).
Malbim: Until now, their faces were towards Yerushalayim when they prayed.
What is the meaning of "Nilvu"?
Radak: It is a command in the past conjugation. The same applies to "v'Nikbetzu" (Hoshe'a 2:2).
Radak explaining Targum Yonasan: It is future in the past conjugation. [There] will come and be added to Hashem's nation, and an eternal Bris will be made, which will not cease. I.e. converts will accompany them when they ascend from Bavel. And so it says in Yeshayah (14:1) "v'Nilvah ha'Ger Aleihem." There, I explained that half the verse discusses ascending from Bavel, and half discusses in the days of Mashi'ach.
Malbim: Tziyon says to [the returning exiles] 'come, let us accompany Hashem.'
