Are you saying the universal church is represented only by the Aaronic priesthood? What are royal priests & presbyters? Is there a difference?
Only? Hardly. The N.T. is the fulfillment of the O.T. God's people in the O.T. were the Jews- & salvation is of the Jews. God's plan of redemption was that Jesus was also to be "a light for the Gentiles." (Isa.42:6;49:6; Lk.2:32) The problem I see with a system of interpretation of historicism(a form of futurism) is that the only conclusion is replacement theology, whereas with preterism, God's promises to Israel are fulfilled & the Gentiles are brought in (Jew & Gentile at that time) in the 1st century, bc preterism is a system of interpretation of of prophecies based on the promise of a new COVENANT to is of the O.T. So, preterism is covenantal theology, not historical (or partial-preterist)
And "presbyter" may be considered as "elders" of a church, royal priests may be both the saints in general & a term to describe an elder or deacon too (1Pet.2:9)
Why would scripture say the Lord of hosts would reign in Mount Zion, and in Jerusalem? What's the difference?
Mt. Zion or Zion is a prophetically loaded term. The perfect state of Zion, as depicted in Isa.51 & 52, implies that the garden of God was at that time considered as Zion. Then God moves on to show that He intendeds to restore to Zion all that was lost in Adam. Zion was lost in Adam- & in Isa.52, God said He intends to restore Zion through his Son. (see Rom.5:10-18). Therefore, through Jesus Christ & his blood, Zion was redeemed- & in Zion there is no "condemnation" or judgment (Rom.8:1) A state as it was in the garden. By obedience to Christ, God has created us anew, like Adam(pre-Satan time) into a safe state. (Rom.6:1-12).
Also, Abraham was promised the redemption of Zion through his seed & that his seed would enjoy the benefits of redeemed Zion. Note this: for something to be redeemed- it must be lost- at the time God made that promise to Abraham.
This means the church was lost by Adam & restored by Christ. Christ not only shed His blood for you & I but for Adam as well.
Is "Jerusalem" any different from "Zion?" Not spiritually or prophetically (as the New Jerusalem) But we can see that the term "Jerusalem" implied "Israel" & Rev.21 is directed mostly to the saved of Israel (the remnant) the "Jew & Gentile in Christ Israel. John was a preacher mostly to the "circumcision) & Rev.21 describes the "no more tears" attending those faithful - in relationship to the tears they shed at the end of the age- but in Christ they find peace.