Where is the exact cut between the regions considered to be Nibenay and Colovia? Colovia is to the west and Nibenay to the east, but where do cities near of the center of the province fall? Would it be a blended culture? What are some of the identifying aspects of each culture?

Skingrad, Chorrol, Kvatch, Anvil, and Sutch are Colovia. Everything else is Nibenay.

Originally, I believe the entire shore of Rumare was meant to be tropical, qualifying it as Nibenay. In Oblivion, Nibenay doesn't exist, so you can just draw a straight line down the middle.

http://images.uesp.net/3/35/OB-regionMap-All.jpg

Brown, sky blue, orange, and light green are Colovia, everything else is Nibenay.