While I understand the popular acclaim you mention, where is there a record of such acclaim in our Church? Are there any parishes in the Orthodox nations dedicated to Augustine? There may well be some, but I have never heard of any (outside of the Western Rite in the west).
I don't believe St Stephen the Protomartyr has ever been formally canonized, and I am not aware of any churches dedicated to him, and yet he is a saint.
Regardless of popular acclaim, there is still a process of Glorification/Canonization that our Church follows, otherwise I could refer to Fr. Seraphim Rose as St. Seraphim of Platina. If Augustine was not formally canonized until after the schism, and then only by the RCC, how then is he a saint of the Church?
There was no process in the fifth century. I don't know exactly how the process arose, but all of our early saints were "glorified" through popular acclaim.
Fr. Seraphim would indeed have to be formally glorified, but today we have that tradition. In the past, they did not. There was never a time where we went back and vetted and glorified all the early saints.
If our Church were to canonize a pre-schism person next week, would the RCC add him to their calendar?
I am not precisely sure that's what happened, though. Why would we add someone to the calendar just because Rome did?
As I stated, I do not support the website that I cited on most of its stances (for instance it condemns my Church, the Serbian, of "ecumenism"), however the article on Augustine has some useful information I feel. There was no Feast of St. Augustine in any of the ancient Orthodox Church calendars as far as I am aware.
There is no feast of St John of Shanghai on any calendar except ROCOR, but nobody doubts his sanctity. Perhaps St Augustine was not considered important in any of the Eastern Churches. Perhaps there were questions about his theology. What is unquestionable is his holiness and his repentance, and that is ultimately all that matters.
I think the uber-traditionalist Orthodox do have important things to say, but their words should be taken with a grain of salt. At least four Churches have St Augustine on their calendar, and whenever he was added, I am sure that our bishops have vetted him and are satisfied. We don't accept his theology for the most part, but that does not disqualify him from sanctity or veneration.
I have the Serbian Church calendar and Augustine is not listed.
Really? St Nikolai lists him in the Prolog. He acknowledges "extremes" in his writing, but again, lists him among the saints for his repentance.