1)
Will David be the king?
1.
Radak #1: Melech ha'Mashi'ach will be called David, for he is from the seed of David.
2.
Radak #2: Yes! This is a hint to Techiyas ha'Mesim.
3.
Malbim: It means that the kingship will return to Beis David, both regarding conducting kingship, and also conducting Torah law.
2)
What do we learn from "v'Ro'eh Echad Yihyeh l'Chulam"?
1.
Malbim: They will all go in My Mishpatim; there will not be discord about Halachah.