Why does it say "v'Atah"?
Rashi: Do not slacken due to this (that Bayis Sheni is nothing compared to Bayis Rishon). Also this Bayis will be esteemed!
What should they do?
Radak: This refers to the coming verse - "Es ha'Davar Asher Karati..." Malbim - do the Torah and Mitzvos over which I made a Bris with you when you left Egypt - "va'Yikach Es Sefer ha'Bris va'Yikra b'Aznei ha'Am" (Shemos 24:7). If so, "Gadol Yihyeh Kevod ha'Bayis ha'Zeh ha'Acharon Min ha'Rishon" (9).
Why does it say "Ki Ani Itechem"?
Malbim: This is a Ma'amar Musgar (parenthetical comment). I am with you now, like I was with you at the time of Yetzi'as Egypt [on condition that you guard Torah like you did then].