Where else would the place be on Usenet? I'm flattered but I'm not one
of the experts who write the books. Answers are strictly personal
interpretations:

In the continuosly diglossic situation, for *written high-register
Byz., the measure of distance is a little like that of late Latin in
Romance lands, ie like asking the distance from the Ciceronian model. A
good measure of the distance from the *spoken language of the educated
is also like in Romance lands for Cic. Lat. : the frequency and kind of
mistakes and of stylistic bloopers. A continuously increasing distance
over 2000+ years.

A precise measure for the written-to-spoken language is not really
available. More analogies in this with the Romance West: Just as
Romance comes from Vulgar, not Ciceronian, so does Modern Greek come
from the Koinê, not direct from Attic, and a lot of what we think we
know of speech is asterisked --reconstructed. True, the papyri, NT,
etc. help for the early language but apart from some occasional
literary outbursts in folk language you have to wait a very long time
until popular poetry gets in full bloom. Even then, heavy influence of
the written high language is always present.

There sure was a common high dialect for the educated. As for speech,
the peripheral areas did develop dialects that diverged over time, some
a lot, to the point of becoming non-inter-intelligible like Pontic or
Cappadocian. But again, there is a lot that is reconstructed / inferred
with the help of the late, recorded dialects. Crete is an exception,
having produced a popular literature from the 15th C or so.

With all that diglossic confusion we also get some confusion on what to
call what.
The modern standard is basically demotic Greek --the continuation of
the old spoken language-- heavily influenced by a century and a half of
compulsory education in Katharevoussa and The War On Loanwords. It
finally looks as the end of 25 centuries of diglossia: The
"Kathomilouménê" or spoken standard is general, Demotic proper is not
used, local dialect regresses fast and merges with standard (with some
regional standards), and Kathraevoussa is dead and buried with no
flowers on its grave (except for the Church and some traditionalists).

Pardon my partly unorthodox interpretation of reading the texts: The
Church then suddenly became the guardian of the "Millet", or Greek
nation under the Ottomans. The only central point with no lay
counterpart, and practically only dispenser of education. Church
language was already strongly marked by the non-Attic NT Greek. Also,
while the upper clergy was highly proficient in Ancient Attic, most of
the clergy was uneducated. We see the written language get the
characteristics of Katharevoussa from then on (spoken language
substratum but with mostly mechanical replacement of many parts by
features of the Attic grammar of the schoolbooks, with set phrases from
antique authors and the NT thrown in, and words suspect of Turkish
origin carefully replaced by one from the old dictionary).

In lands that remained in other hands (Venetian) spoken language
literature finally comes out, free from the pressures of the Empire,
the Church and the School .

Hard to separate Achaea from the rest of the Peloponnese, and hard to
ignore the huge role of Epirus and of the Albanophone Greeks. Prospects
and ambitions were not so much linguistic or cultural --see the sizable
proportion of Albanian speakers-- as they were religious. Greeks nearer
to Constantinople were more efficiently repressed and often a minority
in their settlements. Hard to judge ambitions and aspirations with such
practical obstacles.

Only if "modern Greek public life" includes myth and the occasional
madman. But of course, it's still the see and seat of the Patriarch and
important for the religious-minded. Way more important in cultural
references than any other place.  But back to language: With only very
few Greek speakers left living there, the spoken dialect is
dying-to-dead.