Thank you for your thoughtful questions. Let's talk about them.
You are right that Micah 5.2 doesn't mention a messiah (using the Hebrew term). It says that a ruler of Israel from the line of David will be born.
"Bethlehem Ephratha" is both a town and a clan reference. Bethlehem was a small town about 6 miles south of Jerusalem, but the double place name ("Bethlehem Ephratha") reflects a tribal (clan) distinction within the Bethlehem community. The reference is linking this ruler to David's household (and therefore David's clan of Judah) rather than to a geographical site such as Jerusalem.
The reference gives insight into the evolving clan structure of Israel in the 8th c. BC. "Ephrathah" is a clan name claimed by some of the inhabitants of Bethlehem. The town and clan are not the same, although at times one was used as shorthand for the other. David is repeatedly referred to as an Ephrathite (1 Sam. 17.12; Ruth 1.2, 4.11) to show a clan affiliation.
The prophecy is that this ruler would be born in Bethlehem of the clan of David.
Micah says this king will drive back Assyria, which is an odd reference. Since Micah was writing in about 700 BC, we would expect him speak of the king conquering Babylon. But there was no immediate fulfillment of this prophecy, so we look to some future fulfillment to which Micah must have been referring. But since the prophecy of "David" domination was unconditional, the prophecy is surely for the future since it was not for the present.
Therefore the references to Assyria are figurative. Israel never conquered Assyria. Since the northern kingdom of Israel had recently been exiled by Assyria, it was a powerful image and symbol of nations hostile to God and oppressive to God's people. Such a prophecy pertained to the distant future, and "Assyria" was now an archetype. (The prophet Zechariah also used "Assyria" as an archetypal symbol.)
Then the text mentions "whose origins are from of old, from ancient times." This is yet another reference to a fulfillment other than in Micah's context. Though David's dynasty will eventually come to an end, it will rise again in this ruler. A legitimate fuller sense and deeper meaning of the prophecy is that this ruler will be associated with God from the beginning of time (Ps. 2.7; 110.1). It gives the definite sense of a ruler who transcends time—who has sprung from eternity.
Though the term "messiah" is not used, the Jewish scholars and rabbis understood it as a messianic prophecy. There was a common Jewish conclusion that the Messiah was to come from Bethlehem and the seed of David (Matthew 2.5; John 7.42).
So your reference to and conclusion that the text really refers to Zerubbabel has much less to support it and far more problems that plague such an interpretation.
1. You reduce the text to only pretending.
2. Zerubbabel was not born in Bethlehem, but only of the clan of Judah (Ephrathite).
3. Zerubbabel was not a ruler of Israel but instead governor of the small province of Yehud.
4. I believe that your mention of Zerubbabel defending the land against Assyrian attacks in the days of Ezra and Nehemiah is anachronistic and false. The Assyrians were a non-entity by then. You'l have to substantiate this claim.
Therefore the case for Micah referencing the Messiah is stronger than the case for Zerubbabel.