Why did he say "v'Atah"?
Malbim: The more that Hashem benefits a person, the more he must serve Hashem and be humble in front of Him. This is why a Kohen Gadol bows at the end of every Berachah [of Shemoneh Esre]; a king, once he bows, he does not straighten up [until the end of his Tefilah]. A king must have a Sefer Torah with him, and read it every day. All of Yisrael received Eretz Yisrael - they must fulfill Torah to remain in the land.
Why does it say "Chol Yisrael" and "Kahal Hashem"?
'Rashi': This includes also converts.
What is the meaning of "uv'Aznei Elokeinu"?
'Rashi': In front of Hashem. This is like "Atem Nitzavim ha'Yom Lifnei Hashem Elokeichem."
To whom did David say this?
'Rashi': It was to all of Yisrael. The coming verse addresses Shlomo.
