The OP mentions the controversy with the Donatists and also asserts that Augustine believed in common Romanist tropes about the authority of the church above the scripture.
Interestingly, when Augustine actually debated with the Donatists, he did not do so the way Roman Papists do, always thumping their chest about their authority, but directly appealed to the authority of scripture:
“The question has been proposed: Is the Church of Christ among the Catholics or among the Donatists? This needs to be determined from specific and clear citations in Holy Scripture. First, evidence is brought forth from the Old Testament and then from the New Testament.” (Augustine, Introduction, On the Unity of the Church. My emphasis)
. . .
“But, as I had begun to say, let us not listen to “you say this, I say that” but let us listen to “the Lord says this.” Certainly, there are the Lord’s books, on whose authority we both agree, to which we concede, and which we serve; there we seek the Church, there we argue our case” (Chapter 5). (My emphasis)
Webster says that Augustine basically says,
“Since both parties adhere to the truth of Scripture and believe them to be the word of God, it is scripture which should be the final arbiter.”
Augustine writes, “just as this doesn’t need an interpreter” several times in his appeal to the Donatists. Augustine believed that theses Scriptures were clear and perspicuous, and did not need an infallible interpreter to settle the dispute.
In one of his sermons Augustine gives this exegesis of the rock of Matthew 16:
“Remember, in this man Peter, the rock. He’s the one, you see, who on being questioned by the Lord about who the disciples said he was, replied, ‘You are the Christ, the Son of the living God.’ On hearing this, Jesus said to him, ‘Blessed are you, Simon Bar Jona, because flesh and blood did not reveal it to you, but my Father who is in heaven. And I tell you’...‘You are Peter, Rocky, and on this rock I shall build my Church, and the gates of the underworld will not conquer her. To you shall I give the keys of the kingdom. Whatever you bind on earth shall also be bound in heaven; whatever you loose on earth shall also be loosed in heaven’ (Mt 16:15–19). In Peter, Rocky, we see our attention drawn to the rock. Now the apostle Paul says about the former people, ‘They drank from the spiritual rock that was following them; but the rock was Christ’ (1 Cor 10:4). So this disciple is called Rocky from the rock, like Christian from Christ. Why have I wanted to make this little introduction? In order to suggest to you that in Peter the Church is to be recognized. Christ, you see, built his Church not on a man but on Peter’s confession. What is Peter’s confession? ‘You are the Christ, the Son of the living God.’ There’s the rock for you, there’s the foundation, there’s where the Church has been built, which the gates of the underworld cannot conquer”
(John Rotelle, O.S.A., Ed., The Works of Saint Augustine (New Rochelle: New City Press, 1993), Sermons, Volume III/6, Sermon 229P.1, p. 327).
http://beggarsallreformation.blogspot.com/2014/12/augustines-unity-of-church-finally.html
Also, on the concept of purgatory, it appears Augustine speculated on the topic early on, but then later denied it. On the “eucharist necessary for salvation,” Augustine did not believe that participating in the Lord’s table granted salvation. He believed that salvation was given through faith in Jesus Christ which spiritually fulfilled the command to eat Christ’s flesh and blood. He did believe Christians were obligated to be baptized and to attend the Lord’s supper physically, but only because these are commands, not because he thought that the physical act of itself was effectual to salvation.
““But the sacrament of baptism is undoubtedly the sacrament of regenation: Wherefore, as the man who has never lived cannot die, and he who has never died cannot rise again, so he who has never been born cannot be born again. From which the conclusion arises, that no one who has not been born could possibly have been born again in his father. Born again, however, a man must be, after he has been born; because, ‘Except a man be born again, he cannot see the kingdom of God’ Even an infant, therefore, must be imbued with the sacrament of regeneration, lest without it his would be an unhappy exit out of this life; and this baptism is not administered except for the remission of sins. And so much does Christ show us in this very passage; for when asked, How could such things be? He reminded His questioner of what Moses did when he lifted up the serpent. Inasmuch, then, as infants are by the sacrament of baptism conformed to the death of Christ, it must be admitted that they are also freed from the serpent’s poisonous bite, unless we willfully wander from the rule of the Christian faith. This bite, however, they did not receive in their own actual life, but in him on whom the wound was primarily inflicted.”
(On Forgiveness of Sin, and Baptism, 43:27
I see the misrepresentation of St Augustine continues. he believed as all Christians have for 2,000 years in baptismal regeneration. the reader as the above quote clearly shows. he was a CATHOLIC BISHOP, we should not be surprised he believed and taught the CATHOLIC FAITH.
It is not worth my time to refute the other misrepresentation, but the reader has been put on notice. deception abounds.
Saint Augustine, Doctor, (died A.D. 430): “No man can find salvation except in the Catholic Church. Outside the Catholic Church one can have everything except salvation. One can have honour, one can have the sacraments, one can sing alleluia, one can answer amen, one can have faith in the name of the Father and of the Son and of the Holy Ghost, and preach it too, but never can one find salvation except in the Catholic Church.” (Sermon to the People of Caesaria
the above is what St Augustine taught and believed.