Why does it mention Tziyon?
Radak: It was the primary [place of] kingship.
Why does it say "Nichem Hashem Tziyon"?
Radak: Past tense used in place of future; this is common in Nevu'ah.
Malbim: Also you should hope, for He will console Tziyon, which is now a forsaken widow.
Why is the consolation of Tziyon right after the verses say to look to Avraham and Sarah?
Radak (2, citing his father): When both of them despaired of having a son, in their old age, I gave to them a son. So Yisrael, after their exile extended greatly, until they despaired of Ge'ulah, then Hashem will take them out of Galus.
What are "Kol Charvoseha"?
Malbim: The ruins of Eretz Yisrael. They are Charavos of Tziyon; they accompany her.
What is "v'Arvasah"?
Rashi: Also this is an expression of a Midbar, like "b'Eretz Aravah v'Shuchah" (Yirmeyah 2:6), but Aravah already was settled, and it was destroyed.
Malbim: Midbar is a desolate land. Aravah is a place where thorns, thistles and bushes grow. Aravah is in a Midbar, like gardens and planted places in settled areas. The Midbar produces such vegetation.
How will He make the Midbar like Eden?
Malbim: This is a metaphor; the Nimshal is, the place desolate of people will become a choice settlement, and a place settled with evil idolaters will be a settlement of Tzadikim.
What is "Todah"?
Rashi: It is the voice of thanks.
Why does the verse separate "Sason v'Simchah" from "Todah v'Kol Zimrah"?
Malbim: Places that were desolate will have Sason v'Simchah. Where Resha'im used to dwell will have Todah v'Kol Zimrah.