1)
What is the meaning of "Makeh Yevusi ba'Rishonah"?
1.
Malbim: He will be first to defeat them in war.
2)
Why did he say "l'Rosh ul'Sar"?
1.
Malbim: One can be a Sar, but not a Rosh, e.g. Yo'av. He was Sar Tzeva (the head general), but he was not the Rosh over Yisrael. Therefore, since Yo'av was already a Sar, David elevated him to be Rosh over the city (verse 8).
