1)
How will they be "Goy Echad" with "Melech Echad"?
1.
Malbim: Mashi'ach ben Yosef will rule over all of them. Initially they were two nations, for the 10 tribes served idolatry and abandoned Mikdash Hashem. Now all will serve Hashem.
2)
It is written Yihyeh, and we pronounce it Yihyu!
1.
Radak: The Kesiv refers to the Klal.
3)
Why does it add "v'Lo Yechatzu Od?
1.
Malbim: Initially the kingdom was divided in the days of Ish Boshes, and in the days of Yaravam and after him. This will not happen again. When Mashi'ach ben Yosef reigns, Yehudah will be drawn after Efrayim. Later, when Ben David reigns, all will be drawn after Beis David. This is why it says "Od" twice.
