1)
Did Hashem say so to David?
1.
Rashi: "El David" is like Al David (about him). Also the simple meaning is true (Hashem told him, via Shmuel - PF).
2)
Verse 17 mentioned that Chachamim wanted David. Here it says that Hashem chose him. Which is primary?
1.
Malbim: Kingship requires both that the nation wants him, and that Hashem wants him.
3)
Why does it say "Hoshi'a"?
1.
Radak: This is Makor (an infinitive) in place of Oshi'a (I will save). I.e. I said to save Yisrael through him. The same applies to "v'Eschem Hotzi mi'Tochah" (Yechezkel 11:7). Also Yonason translates Hoshi'a as 'I will redeem.'
