1)
What is the comparison "k'Simchasecha..."?
1.
Refer to 35:14:1:2.
2.
Malbim: Like your Simchah over Churban Eretz Yisrael, everyone will rejoice over the Churban of your land.
2)
Why does it say "Sihyeh Har Se'ir"? Har is masculine!
1.
Radak #1: Here, it is feminine.
2.
Radak #2: It refers to the congregation of Har Se'ir. The same applies to "Kol Edom Kulah", "va'Tehi Yisrael" (Shmuel II, 24:9).