The problem with trying to reconstruct Old Slavonic is that the scribes did update the texts with their vernacular. The authorative grammar of Old Slavonic, Lunt discusses this:
That is how the Russian, Ukrainian, Byelorussian, Moravian, Romanian (yes, Romanian), Serbian, Croatian, Bulgarian and Macedonians recensions of Church Slavonic came about (something Lunt also touches on).
Academic linguistics and common speech are two different things, I would say. In the case of Romanians, for example, they were complaining they didn't understand the "Romanian" Church Slavonic.
At the time (as shown by graffitti, remarks by the educated classes, etc.) Greek was the first language of much, if not most, of the population of Rome due to the influx of slaves, immigrants from Magna Graecia, etc. and the outflux of the Latin Roman citizens to govern and colonize the empire. Classical Latin was restricted to the citizens (now a ruling minority) in the city: it was an artificial standard (like BBC English) that even the native stock of the lower classes had trouble with.
Yes, Latin was an upper class thing and Greek was much more common, but, especially among slaves who constituted such a large part of the church, it was not necessarily spoken well
. In this period, the Romans stole a LOT of their slaves from the barbarian areas of Asia, especially among the Phrygians, but also from among the other barbarian groups that spoke their own languages. More importantly, there's evidence that there were several generations in which the use of Greek vs. vernacular was indeed disputed. Can't find the article now, but it was in the Journal of Early Christian Studies
a few years ago.
Aside from any of that -- as you indicated lower in your post -- there is the issue of basilolect. Some Phrygian who probably had a 1,000 word Greek vocabulary, enough to know when his master wanted him to empty chamber pots, dig up the rotting fish that was buried in the yard to make sure it was extra tasty, or scribble same common phrases in the catacombs like "My daughter, rest in peace," didn't understand the subjunctive purpose clause or the genitive absolute used in the Anaphora during the Liturgy, much less the sermon that employed the best in rhetorical tropes and figures.
From the available evidence, it seems the early bishops of Rome were all (except St. Peter) Greek speaking (I think GOARCH published a book on the "Greek Popes"). Things changed only in the time of Pope Victor I, who started the switch to Latin. But then, he came from North Africa, where Greek, as we know from graffitti, comments of the educated, etc. was a rareity.
Yes. It's quite clear the church leadership spoke and wrote educated Greek in worship, correspondence, theological discourse, etc.
Several years ago, I tried running the numbers. I can't remember the exact outcome, but there's no doubt that the numerical majority of Orthodox Christians who ever lived worshiped in a language they did not speak in daily life.
Are you talking the basilolect, the "high" form of the language, or a different, ecclesiastical language altogether?
For the purposes of comprehension, there isn't much of a difference. However, I don't think it's ever become a different
language altogether, either in the case of Greek or the various Church Slavonics -- all that is required is a little study and attention -- but it ain't what people use over the dinner table, especially because most of the faithful throughout the ages have had about a fourth grade education at best.