I don’t think it’s quite true that facing the east was non-negotiable. For one thing, many of the medieval cathedrals have side altars that do not face the east. And in the 400s, some churches had main altars facing the west, as noted by Socrates Scholasticus in 439 A.D.: “At Antioch in Syria the site of the church is inverted; so that the altar does not face toward the east, but toward the west.” source
The Church’s current practice of the priest celebrating versus populum dates back to the reformation and Martin Luther, and no further
I don’t think that’s true either. The Lateran Basilica and St. Peter’s Basilica both date to before the Reformation, and in both of them the priest traditionally faced the people according to the 1911 Catholic Encyclopedia. (Article: Orientation of Churches)
The same article says, “[T]he fact [is] that in the fourth century the celebrant at Mass faced the people.” For footnotes, the encyclopedia cites several German and French resources, and one English one: Monuments of the Early Church by Walter Lowrie. That book, written in 1901 and available here, presents evidence that the priests faced the people during the second, third, and fourth centuries. Pages 43-46 cite archaeological evidence for this, including a chapel from the 300s in which the celebrant’s chair faced the people from behind the altar, and a painting from the 100s depicting the celebrant sitting behind the altar facing the viewer.
The Church’s current practice…is born out of a novel and false theology. Our Liturgy defines our faith, and our faith defines our Liturgy, as the Fathers taught.
First, there is good evidence in the Monuments book that the Fathers often celebrated Mass facing the people. Second, Lowrie presents evidence that the theology behind this was not a false one. It was based on the Communion of Saints. He writes: “[T]he idea which has most fundamentally inspired [early Church architecture] is the communion of saints — the Christian temple is the house of the congregation. This communion was concretely represented by the altar, which by ancient tradition was fixed between the clergy and the people.” (Page 104)
I’m not saying it is bad to have the altar and the priest face the east, I’m just pointing out that the Church has always permitted the altar to face other directions and the priest to face the people in some circumstances. Even the Missal of 1570 A.D. provides that the priest can face the people: “Si altare sit ad orientem versus populum, celebrans versa facie ad populum [In English: If the altar faces East toward the people, the celebrant facing the people]], non vertit humeros ad altare, cum dicturus est, Dominus vobiscum, Orate fratres, Ite Missa est, vel daturus benedictionem: sed osculato altari in medio, ibi expansis & iunctis manibus, ut supra, salutat populum.” (Missale Romanum 1570 A.D., Ritus Servandus, Chapter 5 Paragraph 3. Page scan here.)
St. Charles Borromeo, one of the critical figures in the counter-reformation who cannot be accused of pandering to Protestant influence, said that the altar is ordinarily supposed to face the east, but can face any direction, and the priest can face the people: “[T]he Bishop can decide and permit that [the altar] be built facing another direction, but in this case care must be taken at least that if possible it does not face north, but south. In any case the chapel in which the priest celebrates Mass from the high altar facing the people, in accordance with the rites of the Church, must face west.” (Instructiones Fabricae et Supellectilis Ecclesiasticae Book 1 Chapter 10, De Capella Maiori)