^^^Follow up/clarification:
Anthropod:
I had to reconstruct my post above, because after I had drafted my original answer, I hit some mysterious botton that caused me to lose all my post…and so i had to re-draft–and in looking back, it occurs to me that I mixed some of my answers up. e.g.–confused ‘references’ for ‘inferences’–so here is the revised resposne to this quetion below:
I have a problem with all this. Even assuming you are right in your biblical references, and also that the ark was immaculate (and I don’t remember the bible saying that), why is it that you believe that Mary absolutely positively MUST have this same characteristic?
First, the Bible doesn’t expressly say that the Ark of the Old Covenant was ‘immaculate.’ However, it was designed to contain the Word of God, called for the utmost reverence, as was treated accordingly by the Jews. God seems to have taken great ‘pains’ to describe in very specific detail, how He wanted His Ark constructed, and handled. And Moses repeats these explicit instructions as he sets out to have the Ark constructed–out of the finest wood, and covered in gold…2 poles (not part of the Ark–rather, they slid into slots or such, which were affixed to the Ark) with which to carry the Ark…the priests had to SANCTIFY THEMSELVES, in order to touch the poles–NOT EVEN THE ARK ITSELF!!! (1 Chronicles 15), the Holiest of the Holies, situated in the center of the Temple…etc., etc.
So while not expressly ‘immaculate’–it is certainly described as being treated with the utmost reverence and dignity–as appropriate to the esteemed task of carrying and preserving, the Word of God (afixed to stone tablets). Furthermore, the type of wood called for (and used), as well as teh gold–imply non-curruptability. Hence we get to ‘immaculate’ wrt the Virgin Mary, by following the analogy to its natural conclusion. That is, in order to get to the holiness of the Ark of the Old Covenant, and its non-corruptability–notwithstanding Man’s fallen nature–the Immaculate Conception appears to be the natural bridge. (Not according to me–but according to Sacred Tradition, and confirmed by the Pope/the Magisterium).
Counter question to illustrate the point:
why weren’t the stone tablets simply made of non-curruptible, invinciple material? Why were they to have been protected by, and carried around in, an Ark in the first place?
…and again…what could possibly be up with the story of the Visitation, if not a reference back to the Ark of the Covenant? What an odd story to include in the Bible, if it served no other purpose than to convey that Mary visited her pregnant cousin…
We go straight from Chirst’s infancy, to his ministry–with but a detour for the ‘lost and found in the Temple’ episode. So some 28 years of nothing…but the Holy Spirit figured we needed to know that Mary visited her cousin Elizebeth… THAT, was worth noting

Seems a bit…benign…if not pointless.
…unless that is, we view it in context of the Ark of the Covenant.
Secondly, does Mary absolutely have to be immaculate? Perhaps not. But it is the most consistent reconciliation of scritpure, and the Dogma of the Trinity.
***If Jesus Christ is the second person of the one omnipotent triune God…then it is most appropriate that the person through whom He chose to become incarnate–whom should serve as the vessel, the Ark of the New Covenant–should be immaculately conceived, and thereby spared of any stain of Sin. ***
Else we maintain that Our Lord, who was and is–the antithesis of Sin, chose to be born of Sin.