That might as well be some Iranian language at this point in time, there is really no reason to believe it's Elamite. Maybe it's Luri: https://en.wikipedia.org/wiki/Luri_languageIslamic sources cite a language called "Khuzi" being spoken in Khuzestan as late as the 8th century, and Khuzestan was the heartland of the Elamite people.
Not sure about Akkadian, I might be wrong on that one.
They were, that's why there's the Eastern and Western Aramaic branches which have an old separation. Even today, Eastern Aramaic languages are barely mutually intelligible amongst each other. Important religious languages like Syriac, Talmudic Aramaic, and Mandaic are all Eastern Aramaic languages though.
Since when did those 2 branches exist?
That's not really true as both the Romans and Sassanids influenced the region a lot in many ways even when talking about rural areas, in any case I would have assumed the relatively stable border would have create 2 disticntly Aramaic languages areas.Both the Romans and the Sassanids didn’t connect strongly with the populations of these areas, especially in the rural areas. Most of the population continued speaking Aramaic as they did. This was furthered by the way these areas were fought over by the Romans and Persians.