Why does it say "Gam At Tishkeri"?
Rashi: Also according to your grandeur, you will get drunk like [Noa 1 ] from the poisonous cup.
Malbim: With this I console you, that similarly happened or will happen to a great city like yourself. You will drink this cup of consolation, like a mourner, and get drunk from it.
Radak: And Egypt.
Why does it say "Tehi Na'alamah"?
Radak: It will be as if she never existed. This is like "v'Shasu v'La'u v'Hayu k'Lo Hayu" (Ovadyah 1:16). It should say Tehiyi (one Yud is omitted). Alternatively, Tehi is third person - verses often switch between second person and third person.
Malbim: You will disappear, as if you are not still in the world.
What is the meaning of "Gam At Tevakshi Ma'oz me'Oyev"?
Radak: Amidst fear of the enemy, you will seek a fortress to be protected in it.