@James Bejon
There are a few aspects I think you are neglecting to consider.
1) Evidence. There is no evidence of Tiberius' reign, or governorship (hēgemon), being portrayed as beginning in any year other than 14 or 15 CE contemporaneously with the reign itself. In other words, neither Tiberius, nor any historians contemporary to him, ever mentioned his reign from an earlier starting point. Neither do these historians give a date other than 14 CE for the co-princeps. All the contemporary physical evidence we have shows his reign portrayed as beginning in 15 CE. And this even rules out ante-dating, which was a practice instigated by the rulers themselves to extend their reign to an earlier point for some sort of political advantage. Tiberius didn't do any such thing. There's just no evidence to suggest that his reign should be counted from an earlier point.
2) Intent. Recognize that even the early church fathers you mention, though they have Jesus showing up at different points in Tiberius' reign, they are still counting Tiberius' reign from his succession from Augustus. Ergo, they are counting his reign as anyone would count his reign, differing only in the calendar system they are using. There is no reason to believe Luke is doing otherwise. Luke was trying to make clear the date, not obscure it with cryptic dating. When he said the fifteenth year of his reign, he meant the fifteenth year of his reign. He did not mean the obscure version of his reign from three years earlier that no one else in the Roman Empire counted from.
In practice, it's like me dating something to 2003, but because you have some theory you want to prove, you go on some long explanation as to how I
actually meant 2000, due to the fact that Jesus was born in 3 BCE, so our calendar is actually behind by three years. But I assure you, if I dated something to 2003, I would mean 2003, with a full expectation of people knowing that I meant 2003, and not, secretly for the well-learned, 2000 due to a calendrical misalignment.
You understand where I'm coming from?
3) Practice and cultural colloquialisms. Understand that Jewish documents were dated according to the year of reign of the current monarch. And there was a very specific method for doing so. Given Luke's constant use of colloquial phrases that would have been readily familiar to a Jew, but not to a Greek or Roman, there's every reason to believe Luke was dating this history in a Jewish manner. And the Jews don't work by ante-dating. Tiberius' reign would have begun at the point of ascension, or 14 CE. For Luke to date the document by the fifteenth year of the reign of Tiberius is a very Jewish way of dating the document.
4) Most of the sources you are citing are two or more centuries removed from the events, and they are making their statements using the scriptures as their source, and their own logic to fill in the blanks.
5) Concerning many of those sources, it is worth informing you that they were also using a displaced calendar system themselves. Please note that the year of the Geminis is not the 4th year of the 202nd Olympiad. Nor is the year ab urb condita 753 equivalent to 1 CE, though the Chronographer of 354, Orosius, Augustine, the Venerable Bede, and a few others all place Christ's birth in that year, for that reason. These writers have their consuls, olympiads, auc years, etc. all out of sync.
So when Christ is crucified when C. Fufius Geminus and L. Rubellius Geminus were consuls in 29 CE, which was the fifteenth year of Tiberius, AUC 782, and OL 201.4, these same sources are calling the year of the Gemini the 18th year of Tiberius, the fourth year of the 202nd Olympiad, and by deduction, the 780th year from the founding of the city. And so, relative to the question at hand, what year are they actually intending to portray? If Christ's birth is off by two years, then so is everything else. Are they determining the start of Tiberius' reign relative to Olympiads, as Jerome and Africanus are wont to do? Are they using consuls and AUC years according to the Catonian standard rather than the Varronian?
I just don't think your source material is as cut and dry as you think it is. That's why I haven't finished that section yet. I'm still researching those aspects. The dates are all sorts of messed up with the ante-Nicene fathers.