This is a helpful survey, Father! I think it would be good to bring out in further relief the point that in the first millennium, kneeling is associated with penitence and mourning, but in the second-millennium West it comes to be associated with adoration (hence the need for the Anglican “Black Rubric” you mentioned). Gabriel Radle’s article “Embodied Eschatology: The Council of Nicaea’s Regulation of Kneeling and Its Reception across Liturgical Traditions” does a good job fleshing this out (Worship, vol. 90, July & Sept. 2016).

The question then becomes: in the ritual language of contemporary Western Catholic Christians, what does kneeling versus standing mean? To me, it seems that the Roman Church has failed in its somewhat half-baked attempt to recover the first-millennium understanding. As another commenter pointed out, if all this is true, then why kneel during the Canon or the Domine, non sum dignus at all? It would be better to be consistent: no kneeling at all on Sundays and during Paschaltide, at any point during the Mass or Office.

But something further should be said: if the modern Roman church takes away certain millennium-old expressions of eucharistic reverence in favor of what has remained Eastern continuity with the first millennium, then it would seem advantageous also to fill up the resulting void with other Eastern expressions of reverence for the Eucharist, such as: Eastward-facing celebration, a templon screen between nave and sanctuary, a communion cloth held under the chin of all communicants, normative fasting from all food and drink from midnight, the private recitation of often lengthy prayers of preparation for Communion, etc.

As an Eastern priest with wide familiarity with the contemporary Roman church, it seems to me that in hearkening back to the first millennium and also to the contemporary East, the modern Roman Church has done little to fill up the vacuum left by so many of its reforms. And so people, looking for a thicker liturgical expression, naturally turn to the way the West itself customarily worshiped up until two generations ago. In other words, this is more than a question of norms and rules; it’s a question of culture—indeed, of inculturation! And culture cannot be changed by force of decree. It has its own enduring ways, and it abhors a vacuum.