Ezekiel 12:13
I will spread my net for him, and he will be caught in my snare; I will bring him to Babylonia, the land of the Chaldeans.
- Does the phrase “Ur of the Chaldeans” imply that Abraham was Babylonian, or more broadly Mesopotamian?
This is a distinction without a major difference. He was not from the city of Babylon but he did come from the geographical area that would later come to be known as the Babylonian Empire, which was in the region of Mesopotamia.
- Is “of the Chaldeans” intended to describe Abraham’s ethnic/cultural identity, or is it a later geographical or editorial clarification for the audience?
The text probably says "Ur of the Chaldeans" to distinguish this city from others named Ur or something similar, such as Urfa, located in today's Turkey. Indeed, the latter city is Abraham's birthplace in some traditions. But in the Bible, the Chaldeans were identical with the Babylonians. (Ezra 5:12, Ezekiel 12:13) So "geographical clarification" is the better answer.
Conclusion: The Babylonian Empire was well known when Genesis was written (regardless of whether one thinks it was in the time of Moses or a few centuries later). Everyone knew it was located in Mesopotamia, and the Babylonian culture was also known to all. There was no need to distinguish the two, so the text is situating his origin geographically, probably to make sure that readers knew it was not the similarly-named town in modern-day Turkey, near where Abram's family settled in Haran.