Why does it say "ba'Yom ha'Hu"?
Rashi: Initially Egypt and Ashur fought each other. Now, those who remain from the plague in Machaneh Ashur, they saw Hashem's hand against them, and they knew that Hashem is Elokim, and they will have brotherhood and friendship with Egypt - also they will know Hashem.
Why will there be a paved road from Egypt to Ashur?
Rashi: People will go constantly from Egypt to Ashur. Radak - there will be brotherhood and friendship between them.
What is the meaning of "u'Va Ashur b'Egypt"?
Rashi citing Targum Yonasan: People of Ashur will fight Egypt.
Will Egypt serve Ashur?
Mahari Kara: No. Egypt will serve together with Ashur; they will serve Yisrael.
Ibn Ezra: Yes, after some of them return.
Radak: No - Egypt and Ashur together will serve Hashem.