... they predate both the Roman and Greek civilizations. If anything the Romans borrowed some stuff from them.
Not so (for the first bit).
The Etruscans were essentially contemporaneous with the ancient (pre-Republic) Kingdom of Rome; the Republic stage of Rome saw the end of the Etruscans as rivals of theirs. The Romans definitely borrowed from the Etruscans (several loan-words have been documented, for example). It may well have gone both ways, but we know a LOT more about the Romans. Etruscan "borrowings" from Rome -- if any (I assert the extreme likelihood thereof) -- are AFAIK lost to the ravages of time and Empire.
Mycenean Greece predates both; then Greece had a fragmented "Dark Age" for some centuries. The "Archaic Greek" era and the rise of the "Polis" city-state model in Greece was, again, roughly contemporaneous with the early Etruscans & Romans.
(I am unclear how much of this historical trivia is relevant to the Dresdenverse; but, see below!)
Jim has been keeping away deliberately from the origins of the White Court, whereas we have been given those of both the Red and Black Courts.
I find that suspicious.
This is... an
excellent point.
Given that we learned relatively long-ago that the Whamp "court language" was Etruscan, the lack of further info is indeed "suspicious" (OTOH, I wouldn't put it past Jim to simply be holding this back in order to provoke speculation, as a red-herring sort of thing).