I appreciate the illumination which your exposition here adds to the understanding of the roles of Mary and Joseph in completing prophesies foretelling and defining the coming of the Messiah.
My view of the "perpetual virginity" is that the hoopla is simply a marketing ploy, one of differentiating a product such that its purveyor, the RCC, can nominate itself as the only outlet for the distribution of supposed benefits to a select and profitable audience in a segmented market. This, of course, presumes that the Gospel of Christ is not freely available to all, but flows only through a channel of "grace" of which the RCC controls the "pinch points."
Any possessor of an MBA thoroughly understands both the context and the methodology of creating a market, uniquely defining the product, nominating oneself as the sole supplier, and branding its offering such that other suppliers will be seen as inferior in service and supply.
It instructive to examine one of the successful patented medicines, acetylsalicylic acid, was patented in 1900 by the German Bayer company, trademarked as "Aspirin" (now passed into the general vocabulary), and marketed as a wonder drug.
At the end of the First World War, the chemical was made generic, available for anyone to make and market. However, through a strong promotional effort, the name "Bayer Aspirin" and vaunted quality was so impressed on the public that for many years Bayer dominated that market segment.
In fact, my mother-in-law, trained as an RN in the 1920s, would accept no other brand of aspirin for her personal use, claiming that none but Bayer made a satisfactorily pure pharmaceutical product, no matter what reasoning she was offered that generic aspirin was much cheaper, and just as effective, even in the 1990s. Why? Because of the continuing persuasive SPAM, no matter how irrational, claimed superior benefits that kept her allegiance even to the grave.
You can argue all you want that from literal-historical hermeneutics a perpetually virgin Mary is nonsense, but you are going to find out that the RCC allegorical approach to the Mary paradigm has such a firm grasp on simony and so much SPAM that it is to Romanists therefore both an honorable and just ploy to cement customers to the brand such that they will never give it up and go back to true New Testament doctrine.
Besides, those Mary statuettes are so pretty that no religious home should be without them. And a Sallman's "Head of Christ." With a populated crucifix and candle beneath it. And a breviary with Rosary beads. Maybe a kneeler.
Too much the case, but many have seen the light, thanks be to God. The amount of labour and contrivances Mary worshipers must go thru in order to attempt to provide a semblance of support for such traditions testifies to the fact these teachings did not become established due to the weight of Scriptural warrant, and that this is not the basis for the veracity of Cath teaching anyway. And because it is not, it also that testifies that Catholicism did change the Bible, as it would not have been hard to at least add a word of special praise to Mary in the espistles to the churches, and or a word about her perpetual virginity, or at least just remove the "till" from "And knew her not till she had brought forth her firstborn son: and he called his name JESUS." (Matthew 1:25)
But since that was not practical, and because Truth is only what Catholicism says it is, then that did not happen.