I agree that Latin influences on Greek were probably few, if any. But that is not the result of Greek being stronger in some way than German or Gallic. Rather it is the result of Greek being considered a mark of culture among the Roman upper crust. All well-educated Romans spoke Greek as a matter of course; it was a mark of breeding and refinement. Even Caesar’s famous last words were spoken in Greek. One result of this was that the Romans tended to use Greek rather than Latin when they were east of the Adriatic. Even the early Church tended to use the local language for mass until the 10th or 11th century. The imposition of Latin was one of the many factors that drove a wedge between the churches of east and west that are today the Catholic and Orthodox churches. Actually, if you look at the line defined by the boundary between the two churches, you can pretty much see where the Romans switched between Latin and Greek.

As for ancient Greek being largely intelligible to modern Greek speakers, well, true there was some continuity over the millennia, but the connection is a bit more artificial. After Greece regained its independence in the early 19th century, there was a deliberate effort to reintroduce Greek as a living language. The new government went to a great deal of trouble to develop a form of Greek closely based on Classical Athenian and the Hellenistic Koine. (The Norwegians did something similar after their independence from Sweden.)

Now, I did a bit more looking and it seems that my earlier comments on alphabets were not quite accurate. In fact, the whole thing is rather complicated. Essentially, local alphabets are divided into two classes: “red” alphabets and “blue” alphabets. The difference is based on the sound values applied to certain letters (but there is overlap) and the division is roughly east/west, with “red” alphabets in the western colonies and mainland and “blue” alphabets in the eastern colonies and mainland. Basically, “blue” alphabets used chi (χ) for ‘kh’ and psi (ψ) or phi sigma (fs) for ‘ps’. “Red” alphabets on the other hand used psi for ‘kh’ and chi (or sometimes chi sigma) for ‘ks’ (which explains why the Latin ‘x’ looks like the Greek chi).

Some other differences: Corinth used a “blue” alphabet, but used the obsolete san for ‘s’ rather than the sigma everyone else used. The Attic alphabet only ran from alpha to chi until the end of the fifth century BC. Epsilon (e) was used for all forms of ‘e’ and omicron (o for all forms of ‘o’. Eta (h was used to indicate rough breathing. Then the Athenians adopted the East Ionic or Milesian alphabet in 403 BC and it ultimately led to today’s modern Greek alphabet.

From all of that I assume that the Spartan/Lakonic alphabet was “blue” and thus resembled what we think of as the Greek alphabet, but there were probably some differences, too. I trust BRASIDAS is now thoroughly confused.