It's understandable that there's not so much divergence from Mažtane going far to the south-east as there is going into the more immediately distinct mountainous areas (Barakhûn/Ismahi). I know the river trade encouraged cultural continuity, and imagine pilgrimage to Como played a part in this too. But with Aranicér's former importance, details like the southern pantheon, the fairly long history of political independence and even simply the distance I'd not have been surprised to have found another separate Central Eastern language lurking down here. (Or several.) Is there, perhaps, a history of surpression of localism dating back to the empire, that might help to explain this? Or is it just down to economics?
[Edit: I'm thinking as a comparison point about the classification of Scots as a separate language rather than dialect of English, which it seems was obvious in the medieval period, down-played and surpressed when it was politically convenient - during which time it also arguably moved closer to Scottish English such that the classification was more objectively controversial, and is now back in vogue.]
As I was getting interested in all this I noticed the following in the Reference Grammar:
"Note: the dialect descriptions in the old grammar are extremely outdated. They will be replaced with descriptions on Almeopedia."
Well, I'm looking forward to it!
(Or perhaps I missed it already!)