1)
Why does it say that his son will build the Beis ha'Mikdash "Lishmi"?
1.
Malbim (here and in Melachim I, 5:17): Since his son will have serenity, he can build it l'Shem Hashem. David, who did not have serenity, could not, for he would profit through it, like Nasan said that building it will bring rest from enemies.
2)
Why does it say "v'Hachinosi Es Mamlachto" in verse 12, and v'Chonanti Es Kisei Mamlachto" here?
1.
Malbim: Hachanah is the start of the kingship, and Konenus finishes the building of it.
