I'm probably alone on this but I'm thinking Naomie Harris could play Coretta. They have very similar facial structures. I don't know maybe I'm probably the only person who sees it, but I think it would be a fine choice.
Also little factoid, I go to school with MLK's Great Nephew's daughter. She's extremely nice. Although I do believe her father died a couple years ago. He was in very bad health but he was such a great guy. Her mother is equally wonderful as well. Such a lovely family.