Eureka

After a lengthy search, I have found the answer I was looking for:

 The chief objection raised by the heretics to the virginity in partu is that, in the eyes of its adversaries, it makes our Lord's human birth and thus His human nature itself seem unreal. Does the doctrine not betray Gnostic or Manichean disdain for the flesh? Was it not a Gnostic, Valentinus, who taught that the Son of God merely "passed through" His Mother, as through a channel? (14)

In reply to this objection, we must again invoke the distinction made within the Tradition between what Christ is as man and how He comes to be man: as St Leo says, just because His conception and birth (how He comes to be man) are miraculous, it does not follow that His human nature (what He is as man) is dissimilar to ours. (15) In the manner of His human birth, says St Thomas, Christ wants to reveal the truth not only of His humanity but also of His Divinity. That is why "He mingled marvelous things with humble ones. Thus, to show His body was real, He is born of a woman, but to show His divinity, He is born of a virgin, for, as St Ambrose says in his hymn on the Nativity, 'Such birth befits the God of all.'" (16) The heretic Valentinus denied that the Son of God took anything from His Mother, whereas the Church confesses that He is man "from the substance of His Mother," (17) that His flesh is fashioned by the Holy Spirit from His Mother's pure blood. The virginity in partu is a miracle of the bodily order, a cherishing and beautifying of the Virgin's flesh. Such a miracle would be of no interest to the Gnostics or Manicheans, who despised the body and sought for it no splendor. The preservation of virginity in partu manifests a God who not only creates the biological realm but also descends to its depths in person. Our Lady's virginity is a quality of her soul as well as of her body. But the rational soul is the substantial form of the human body, making it to be what it is, the body of a human being. It is therefore fitting that its beauty should be manifested through the beauty of the body. We could even say that the virginity in partu is a kind of divinely instituted sacrament of the virginity in Mary's soul. The matchless maidenhood is both corporeal and spiritual. As St Bernard says, "She was a virgin in body, a virgin in mind, a virgin in profession, a holy virgin in spirit and body." (18)

But why was it necessary for the Son of God to be born as man in a way that would not injure the integrity of His Mother's virginity? The necessity is again one of fittingness, of harmony and thus of beauty, like the need to fit a third and a fifth alongside the root to achieve the lovely consonance of a major chord. The virginity in partu is what is needed for Christ's human birth to be in accord with the rest of His work in creation and redemption.

Fr. Saward goes on to explain how His birth accords with the rest of His work of redemption. Read it all and see if you agree whether vaginal delivery in keeping with Mary's sacred embodiedness contradicts this dogmatic mystery. It does vindicate my skepticism, while it certainly confirms that JPII wholeheartedly accepted this belief in the virginal integrity; and he is the source for TOTB. I wish he had written at greater length about this topic. I'm still willing to plunge into uncharted territory, but this article might be a good tether to tie around my waist as I go.