Who are the guards?

Rashi: They are angels.

Maharal: Yerushalayim must be guarded due to its great level. When it is settled, it has a supreme guarding.

Maharsha: The angels advocate merit for Yisrael, and supplicate that they be redeemed soon. The verse discusses at a time of Churban, for it says afterwards "Ad Yechonen."

Do they always say "Atah Sakum Terachem Tziyon"?

Maharsha: Yes. Every day is proper for redemption - "ha'Yom Im b'Koli Sishma'u." This is why it says "Boneh Yerushalayim" in present tense.

Why does it say "Boneh Yerushalayim Hash-m Nidchei Yisrael Yechanes"?

Maharal: Yerushalayim can be built only via Yisrael. It is in the middle of the world, therefore, it is special for gathering.

What is the significance of "Ki Vachar Hash-m b'Tziyon [Ivah l'Moshav Lo]"?

Rashi: Also in the future, the angels will guard and say that Hash-m chose Tziyon.

Iyun Yakov: Rashi learned from the Reisha of the verse "Zos Menuchasi Adei Ad" - in the future, there will be no further Galus and Churban.

Maharsha: He chose it in this world, and desired it for a fixed dwelling in the world to come. The letters of "Ivah" hint to this. (Now, "Yad Al Kes Kah - Kisei lacks an Aleph, and Shem Havayah lacks Vov Hei. In the future), the Kisei and Shem will be complete.