Be warned it is a long read. I would focus on part LVI
The Holy Tradition of the Apostle's was complex and not all the early saints had a clear grasp on all of it. For example, the clear Trinitarianism we see in St. John and his disciples such as Ss. Polycarp and Irenaeus was not universally present. The early saints were learning the apostolic tradition even as they evangelized. Most of them did not have access to all of the books of the Canonical New Testament. Indeed it was Tertullian who later fell into heresy who coined the word Trinity to help people understand the nature of the Godhood.
And indeed you were correct also to say initially the line between presbyter and bishop was blurred. However by the time of St. Ignatius, it was certainly crystal clear in Antioch, which was the most important Eastern church at the time I think, already having eclipsed Jerusalem, which would soon be mostly destroyed within 40 or so years of his martyrdom. And the epistles of Ignatius ensured this model became standard. Also the Orthodox see in 1 Clement an episcopal model, but this is debated.
The early church must be understood as a church in formation, which by the time of Arius had fully formed as a result of everyone having thanks to St. Ignatius understood correct polity, thanks to St. Irenaeus having understood correct doctrine (except regarding the Millenium; Chiliasm seems to have been an aspect of some the followers of St. John but we have other early saints like I believe St. Justin Martyr who were aware it was wrong), and thanks to Tertullian, having one word to express the Godhead, and thanks to Hippolytus having access to vital information on the Apostolic canons, correct liturgical forms, and more catalogs of heresies, and through Origen, having a formation in philosophy and deep intellectual training.
The result of all of this was that the bishops at Nicea were able to instantly identify Arius as being a heretic. Providentially, God prepared the church in size and strength to receive the huge influx of converts after Constantine following the Diocletian persecutions, which ensured the authenticity of the faith of the hierarchy. Ss. Alexander of Alexandria, Nicholas, et al, actually lived through that, but many of their friends and predecessors were killed. In like manner, the church was prepared to fight off Arianism after the death of Constantine when the political machinations of Eusebius of Nicomedia.
I believe the Twelve Apostles and St. Paul had through their close contact the entirety of Holy Tradition. However it had to be documented, and the Synoptics omitted key things requiring St. John according to our tradition to write one more to supplement it. And then it took a while for this tradition to be fully propagated as you inevitably due to a shortage of manuscripts and the insidious presence of Gnostic and other heretical forgeries, and you needed heresiologists like Ss. Irenaeus, Hippolytus, and Tertullian, before the latter became a heretic, to document the heresies and refute them, and you needed scholars like Tertullian, Clement of Alexandria and Origen to make the tradition easy to understand.
In the fourth century in turn Ss. Athanasius, Basil, Ambrose and John Chrysostom in particular did much work to make the faith ever more comprehensible as a whole, while others such as Ss. Anthony and Gregory of Nazianzus probed the mystical depths. And you had still others like St. Nicholas distinguished through charity, and other types of saints, through whose labor and bloodshed the Apostolic faith, having been preserved and consistently propagated, was now made readily accessible, easy to discern from heresy, and demonstrated in its virtue through charity.