David continues speaking to the Israelites.
One possible meaning is that here David is a metonym for David and his descendants who will rule Israel forever. AT: "chose me and my descendants ... to be kings over Israel forever" Another is that David will continue to be the king of Israel after he is resurrected from the dead. (See: rc://en/ta/man/translate/figs-metonymy)
"the whole land of Israel" or "all the Israelites"
Here "to sit on the throne" means to rule as king. AT: "to rule over" or "to be the king of" (See: rc://en/ta/man/translate/figs-metonymy)
"Israel, which is Yahweh's kingdom"