Actually, this seems to significantly undermine the ground I think you’re trying to take, since the main pronouncements against polygenism come in the form of letters from pope’s not explicitly defined by them nor by ecumenical council as being infallible but, as you say, as a “teaching from” doctrine but within the realm of a pope’s “free speech.”
It has seemed to me that Pius’s statement that it is “in no way apparent” how polygenism could square with teachings of Original Sin and Redemption are actually somewhat weak words–for they leave themselves open to a later explanation that does indeed harmonize the two satisfactorily, within the realm of the development of doctrine.
In fact, it ends up sounding like some of the dispute in the early days of formal Western science and astronomy, even the heliocentrism debates, where scientists were admonished to have extreme caution, because it was not immediately apparent how new discoveries could square with old understandings of Scripture. Galileo got himself into trouble the same way many evolutionists do today–by claiming more than the evidence from Natural Revelation showed, and not properly taking Divine Revelation into consideration. But still, some of his core assertions–those of the more careful and proper Copernicus–were indeed explored, accepted, and grew into a greater realization of an even greater glory of Creation than we had previously perceived, and one that still fits quite nicely with Scripture and Tradition.
So I would caution both sides against a repeat of the Galileo affair–scientists from going too far as Galileo did, and the faithful from just dismissing such ideas as incompatible with faith without sufficiently and continually exploring and seeking harmony between our understandings of Divine and Natural Revelation.
In one sense, it can come down to how you are considering the “gaps.” Are you asserting a “Darwin of the Gaps” in assuming that, while we still don’t know how much happened, the theory of evolution MUST be the solution (a faith-statement in a man-made theory)? Or, on the other side, are we asserting a weak “God of the Gaps” argument by assuming that what is not explained well by evolution is done by fiat miracle, absent observable/discoverable secondary natural causes.
I am aware, as was explained a few pages back, that evolutionary theory cannot at present account for the positive creative information required for the formation of complex new physical structures or species. It seems to explain adaptation fairly well, and some examples of speciation (particularly behavior-based reproductive isolation in the wider definition of “species” commonly used today, rather than true genetic incompatibility). But do we assume that God miraculously intervenes to produce new biological information to achieve this, or does He use some as-yet-undiscovered secondary natural cause to do so?
Any of these requires, ultimately, an Intelligent Designer, so even the naturalistic approach does not get around God.
And this is why I say that arguing about this issue too often expends much energy in antipathy, rather than seeking to truly understand God’s Creation better. We unnecessarily pit our discoveries of natural processes against God or vice versa. Such a complex nature still required a Creator, so there is no inherent threat to faith.
The question of first parents is a little bit different, but even there, as I’ve said before, there seems no conflict with Faith no matter what process through secondary natural causes God used to “form” man’s body out of the dust, for we know, still that He infused (“breathed”) an immaterial soul into that physical body, making a new creation uniquely in His image and likeness.
That doesn’t answer the question of polygenism, but it does seem to render moot any concern that biological evolution of the body is a threat to the understanding of man as body and soul.