Why Augustine left Manichaeanism for orthodox Christianity may never be known. As far as I know, he did not record his reasons. Certainly, he seems to have retained some concepts from Manichaeanism, so he presumably was not entirely at odds with the theology. He may have had a falling out with the Manichaean hierarchy, he may have sought higher status in the Manichaean Church and had that ambition thwarted, or he may have felt that Manichaeanism did not offer what he needed spiritually.

No. There is no mention in the gospels of original sin. The concept of original sin was developed by Augustine of Hippo in the fourth century. He was arguably influenced by the Gnostic and Manichaean notions of this world as inherently evil, but prior to his time there was little or no interest in the concept of original sin.

The Church Father, St. Augustine of Hippo was a Manichaean 'Auditor' from 373 to 382, when he converted to orthodox Christianity.

It was Augustine who developed the notion of original sin, which had been unknown as a central theme of Christian thought.

He saw humanity as almost irredeemably wicked and perverse, rejecting any idea of some innate goodness. To him, salvation is an entirely undeserved act of grace that plucks us from our filthy state of evil.

Aurelius Augustinus (354 - 430), Augustine of Hippo, initially followed the Manichaean religion, then Neoplatonism and finally.converted to Christianity in 386. He abandoned his career in rhetoric, quit his teaching position and devoted himself full time to religion, celibacy and the priesthood.

Augustine became a famous preacher and was noted for combatting Manichaeism and Donatism. It is largely due to Augustine that Western Christianity established the doctrine of original sin, a key source of division with the Orthodox Churches.