There is a flaw in your argument. Original sin is passed on through the race of Adam. It is inherited. Since Mary was part of the race of Adam she too inherited original sin.
If Mary did not have original sin there would have been no need for a virgin birth since it would mean original sin must be picked up some other way besides birth. Consequently, our Lord Jesus could have just been born the normal fashion and not have original sin. No need for the virgin birth.
I think you also make to much of the comparison of the Ark of the Covenant and Mary. The Ark was a symbol for the people of Israel that God was with them-that's all. When they acted badly the Ark was meaningless (1 Samuel 4).
There is a certain irony in your comparison for the people of Israel tended to worshipped the created rather than the Creator. The Ark is one example. The serpent on the brazen stick is another. We should hold Mary in high regards like Esther, Ruth, Paul, Peter, etc. But to worship Mary is equivalent, to use your analogy, to worshipping the Ark or brazen serpent.
I'm not sure what the Reformers positions were about Mary and sin but if they held Mary was sinless this is contrary to what the Bible says and they were wrong. This shouldn't come as a shock to you since you feel they were in error in other places where they ARE supported by scriptures.
I do not believe any of the Reformers held Mary to be sinless (i.e., superhuman). Some of them, like Luther and Calvin, held to the extant view of Mary's virginity, and that the "brethren" of Jesus were cousins or some relative other than half-brothers.
Luther, for example, made rash statements such as, "Christ, we believe, came forth from a womb left perfectly intact." A bloodless birth?? Perhaps, too, a bloodless circumcision eight days later. The problem is, when you study the Reformers you find no Scriptural support for their wild suggestions. They were men, prone to error, as all men are. Let's not make them out to be protestant popes. That's why we can only trust Scripture alone as the ultimate arbiter of divine truth.
I do not believe they had any good theological reasons for maintaining that view. And remember that they lived long before the extreme beliefs of modern RCism on things like the immaculate conception and assumption were codified by the RCs.
The errors they were dealing with in the church were much more grave than devotion to Mary, although one might argument that Mary devotion was a symptom of the larger problem. They may have a different view if they had witnessed the extremes of Mariology in modern times.