We covered this before. The infant-raised-in-the-woods-out-in-Patagonia is specifically answered by St. Thomas. If such a one truly strives to serve God by obedience to the natural law, God will send an angel, a missionary, or His own direct inspiration to enlighten him in the truths of the Faith necessary to salvation; thus coming to the knowledge of the truth. "God wills that all men be saved, and come to the knowledge of the truth" (I Tim. 2:4).
Would you say then this is exactly what Pope Pius IX was talking about in QCM but perhaps only implicitly and not getting into too much detail? Do you believe he was just echoing St. Thomas or was there something new being said?
Yes. The point is that without supernatural Faith one cannot be saved, not that ignorance of any sort can or can't save, which is a nonsense. If we follow St. Thomas, what we are saying is that a man who hasn't the Faith, whether he is guilty for that or not, cannot be saved. This is
de fide. If he is innocent, God will send him further graces so that he may be enlightened and brought to salvation, which is all that Ven. Pius IX is saying. He is merely cutting off accusations of injustice or "unfairness" against God, having just laid down that there isn't any salvation outside the Church. In other words, "Don't worry about the invincibly ignorant - they won't be left to die in that state. If they are truly innocent they will be brought to salvation by God's light and grace." That is, by His Light - which means, by being granted the light of true Faith. None of this was controversial when everybody followed St. Thomas, but between the liberals and the "Feeneyites", it has all gotten tangled up.
As for whether we know about individual cases or not, that is not really relevant, although some try to make it so. The point is to establish which PRINCIPLES govern the question of salvation. Individual cases will then fall within or without those parameters.
While I agree that we do not know with absolute certitude the disposition of another's soul, we can know with moral certitude. We must avoid the trap of saying that because we do not have absolute or metaphysical certitude we do not have certitude. Or because there are cases we cannot know about, that we can never know about any cases at all.