Waaay after the fact, I know, but I just saw Sophie Okonedo in Doctor Who, and it occurred to me re: your Eko conundrum that she could be great. Oscar nominated as Don Cheadle's wife in Hotel Rwanda, so she could do an accent I'm sure. And she's rather tough and intense when she wants to be.
no subject
http://www.imdb.com/name/nm0645683/