Excellent argument. I subscribe to it myself, except on some of the details at the bottom, namely, the assumption, although I do not deny the piety of it or condemn those who hold it as pious opinion. My objection is purely on the grounds that one must believe it in order to be in communion with a billion Catholics. It seems to me that whatever the merits of and safeguards built into Vatican I's statement on papal infallibility, the two 'infallible' of the pope- both on Mary- served no purpose than to institutionalize personal piety and thus put up further barriers between the church and its erstwhile sympathizes (myself first among them).
But I digress.
Your excellent argument, as you state it, does not show (because it cannot show?) why, however much we acknowledge and perhaps even venerate the Theotokos as the Queen of Heaven, we ought to pray for her intercession in various matters. Prayer venerating her is one thing, but prayer for her intercession is quite another.
Now, Scott Hahn argues in his book Hail, Holy Queen (which I thoroughly enjoyed, by the way) along these precise lines. He writes, in particular, that the Davidic Queen-Mother of Israel received intercessions on behalf of the Israelite people and took these petitions before the king. As with Bathsheba (an unfortunate example susceptible to Protestant polemic, much like the Queen of Heaven in light of Jer 44), so with the Blessed Virgin.
Now, I'm going to skip over a common Protestant argument even though I think it holds some weight. That argument is that we have direct access to God the Father through the theoanthropos and sole mediator, Jesus Christ. I would merely note two things about it: first, contra the Protestants, the Catholics do hold that Christ is the sole mediator, but make a distinction between mediator and intermediary (the merits of that distinction, which has implications for everything from prayers to the saints to the nature of the three-fold ordinate, can be explored elsewhere). But second, contra the Catholics, I would note that, Jesus Christ, truly hearing our prayers, and the Father, truly receiving them from the Son, does undercut the pragmatic nature of the ancient Israelite arrangement. After all, aggrieved persons could not always petition before the king, so the queen-mother was sought after as an intercessor precisely because the king did not have their ear (he was, after all, busy being a king and being entertained by his harem). This pragmatic arrangement is severely undercut by the most basic affirmation about the incarnation: that in Christ all the fullness of divinity- including omniscience- dwells bodily.
Well, that was less of a skipping over than I would have liked...
Nonetheless, I want to grant you that however much the position of a queen-mother intercessor is unnecessary, the arrangement may still stand. Standing, then, are we able to pray to the Queen of Heaven?
Now, the other Protestant argument that I am not just going to skip over, but expressly deny, is that the saints cannot hear us. Well, the saints in general may very well be unable to hear us. Who knows? But if the queen-mother's role as royal intercessor for the aggrieved still stands, than it only stands to logic that she can hear us.
However, this is the limits of what we can know by Scripture and logic, and however much I love holy tradition, that rarely convinces me on a matter, especially when the development of a tradition is not genuinely ecumenical and is fairly late.
So how far has the logic taken us?
First, I contend that this logic only really takes us to corporate intercessory prayer. Individual prayers are unwarranted by the reasoning.
Second, I contend that these corporate prayers are only applicable (perhaps even only heard!) within the context of Mary's role as the Davidic king, and therefore only in the cases of the specific, corporate grievances of the people of God.
Thus prayers for Marian intercession, inasmuch as they should be practiced, should take the form of public and corporate liturgical prayers for corporate and particular grievances, namely, disasters that have befallen Christian communities, especially persecution. That is all I find warranted by the argument.
I can understand your disagreement over the Assumption. On face value it seems like I'm throwing away Jesus' merits and Mary's merits and so forth, but I just wanted to present the Jewish origins of the Messiah's mommy. The fact that Elijah and Moses appeared to Jesus on the mountain represented the fact that Mary would assume into Heaven, at least in the sense that they believed the Ark would be found again in the New Jerusalem and the New Jerusalem would be in Heaven and Israel would be brought into Heaven. There's a lot of contradicting symbolism for the Mother of the Messiah - for example: she symbolizes both the Ark, which is in Jerusalem, and Jerusalem, which is said to house the Ark - but this is due to the contradicting prophecies of the Messiah - for example: the Messiah was suppose to be all at once the poorest man in the world and the wealthiest man in the world.
It is extraordinary that God fulfilled over ten thousand prophecies concerning the Messiah - not to mention all of the prophecies of His Mother! Literally every detail of the Messiah's life, even the most minute, fulfilled a prophecy: His parents' names, where He went, who He chose as Apostles, who He spoke parables to, the fact that a demonic was in a graveyard cutting himself all night, Pontius Pilate judging Him, the fact He was born during the Roman Empire, etc. It would take a minute control of the life, geography, environment, and nation of the Messiah to fulfill the prophecies, including those which contradict one another, and such control could only be done by a supernatural agent: Even the devils would not be able to pull it off. And we call this agent Divine Providence.
You said that a problem with Marian intercession is God's omniscience in Jesus Christ. But the problem with this argument is fourfold.
One: God is the Father, the Son, and the Holy Spirit. We receive everything from the Father through the Son. Yet is not the Father God just as the Son is God? So would not be proper to pray directly to the Father? Or, would it not even be proper to not pray to the Father through the Son but just to the Son? After all, both Persons are One God. But see this is what most non-Catholics don't understand: They mistake the "model" for the "work": They say, "The 'model' is like this, so the 'work' is wrong!" but forget that, because of the "model", the "work" is right. So just because Jesus is God, it doesn't mean He cannot act through Mary.
Two: Israel is the sign and instrument of God's salvation. God willed to bring all nations into Israel, just as He wills to bring all men into the Church, because He chose Israel to be the sign and instrument of His salvation, just as He chose the Church to be the sacrament of His salvation. In other words, God worked through Israel and now He works through the Church. God through Adam created Eve; God through Noah saved humankind; God through Abraham intervened for Sodom; God through Isaac brought forth Jacob; God through Jacob brought forth Israel; God through Moses brought plagues on Egypt and led Israel by the hand out of the land of bondage, gave the Commandments to Israel and pitched His tent amongst men; God through Joshua brought Israel into the Promised Land and conquered its enemies; God through the judges ruled over Israel; God through David, Solomon, and the other messiahs ruled over Israel; God through the messiahs' mothers interceded to the messiahs; God through the prophets reminded Israel of its sins and need for conversion and to live the Torah; God through Mary brought Jesus into the world; and God through Jesus fulfilled His promises and prophecies of old, and He continues His work through the Body of Christ, i.e., the Church. If, than, God works through men on Earth, why not through Mary in Heaven?
Three: The angels of Heaven. In many messianic prophecies, God speaks to His angels, asking them questions. Each time He does, they are baffled, wondering why an Omniscient God would need to inquire of something from creatures? But each time He does, God explains to them that, it isn't because He is not Omniscient, but because He wishes to reveal something to men, and so, work for man's salvation. The same goes for Mary. He could have willed to work another way, but, because He knew in His Wisdom that working through Mary would be to man's best interests and salvation, He so chooses to work through Mary, allowing her to intercede for man to Him.
Four: Mother of the Messiah, Mother of the world. While Mary does intercede at all times, she likewise praises God at all times. She is not so obsessed with man that she forgets God. But because she is His Mother, she is ours as well. Not only because of the universalization of Israel, nor only because Mary is a symbol of Israel, nor only because of the universal Redemption of man, nor only because of the Will of God to save all of mankind from destruction, but, in addition to all this, it is also because Jesus Christ has, within His Humanity, all of us: We are in Him, even while we are yet enemies, in a mysterious way due to the Mystery of the Incarnation. Because of our union in Him, we are all sons and daughters of Mary, spiritually before Baptism and actually at and after Baptism, so that, being children of Mary, we can, just as Jesus did, ask Mary for things. As she changed Jesus' diapers, she changes ours. As she brought Jesus to Jerusalem each year for Passover with Joseph, who led her there, so she brings us to Jerusalem each day for Mass with God the Father, who leads her there. Whatever she did for Christ, she does for us, being an exemplar of the Golden Rule and of those who live by the Will of the Father.