Why does it say "Ki"?
Rashi: Here it means Ka'asher (when). Even when You lowered us, we did not forget the name of our G-d (verse 21).
What is "bi'Mkom Tanim"?
Rashi: It is a Midbar, a place of serpents.
Radak: It is amidst the nations. Those who rule over us are metaphorically called serpents 1 , which are cruel - "Gam Tanim Chaltzu Shad" (Eichah 4:3).
Malbim: We were among people as cruel as serpents.
I do not know why some texts of Radak mention also Ye'enim (ostriches).
What is "Tzalmaves" that covered us?
Radak: It is darkness of Galus.
Malbim: They killed us with bizarre deaths and afflicted us with harsh afflictions to [try to] make us abandon our creed.