An accurate example, and first you must understand that scripture is only a part of the deposit of faith, and it is not the source of all church teaching. Catholics hold the apostles taught more than was written in the books of the Bible. This is what we refer to as Sacred
Tradition. Paul instructs Christians "to hold fast to what has been taught to you either through word of mouth or in a letter."
Remember that Oral teaching was the main vehicle of the early Church as there were no bibles, and the writings of the apostles and their deciples, had little geographical reach, and would not even be declared cannon until the 4th century at the Councils of Rome, Hippo and Carthage.
Your logic is faulty.
Note that, Paul did not except Jesus in the first cited quote. So this "all" is not universal.
Your second quote, does not indicate that Christ is alone in not knowing Sin, and that isn't even the point of the narrative.
Your conclusion is an illogical jump, as Mary's sinless life was a grace afforded her by God, not something she acheived under hew own power. Furthermore it is a belief that has existed from the earliest days of the Church. The Church Father's interpreted the Angel's greeting at the Annunciation, "Hail, full of grace" to mean that Mary was free from the curse of original sin, and therefore capable of living a life free from sin.