1)
Why does it say "Lechu Na"?
1.
Radak: If you think that 1 will not destroy My Bayis due to your deeds - see what I did to Shilo! I destroyed it and removed My Shechinah from it, and the Aron was exiled from it and was with the Pelishtim, and I was not concerned for My honor, since [Yisrael] disgraced Me. So I will do to this Bayis!
2)
Why does it say "Asher Shachanti Shmi Sham ba'Rishonah"?