I should say off the bat that, of all the things I've currently implemented, the Greek overtones of Corinthia are probably the most controversial with the scholars I talked to. Howard had stated he had little to no interest in the Greeks of the classical age, losing interest about the time the Dorians settled, and he outright hated the Romans. Thus, I'm having to be careful about how much Greek/Roman to implement in the mod since Howard was not a big fan of either.
That's an interesting fact I wasn't aware of -- I guess that explains why REH never set any stories in ancient Greece! Though I would suggest that, as important it is to keep to the spirit of Howard's world, it's also important to create a fun game-playing experience, and Total War fans love Greek and Roman units -- so I would suggest balancing what the scholars say with what would result in the most fun possible. To that end personally I'd be inclined to include Greek and Roman elements -- both provide quite unique and fun tactical systems of combat that would juxtapose very well with the more medieval-themed units of most Hyborian armies. Where else but in this mod would a Total War fan ever get to pit a Roman cohort against English longbowmen, or a Successor phalanx vs. French knights?

I think you've demonstrated an uncanny ability to keep with the spirit of Howard's world while also emphasizing elements that you intuitively understand will just plain be fun (say, bands of marauding Snow Apes ) -- clearly you have an excellent instinct for game design, so I would suggest you give your gut feelings as much weight as you do these Howardian scholars...

As for the plans you have for Corinthia, and the various cultural influences, all your ideas make sense to me. And these cultural identities might spill a bit over national borders -- so perhaps some of that 'Thracian' influence extends up into Brythunia, etc?