Charles Eastman?

Yes, he was and was indeed the doctor who attended the survivors of Wounded Knee. However, he was not mentioned in Dee Brown's book, probably because Brown's book ends directly after the massacre, and Eastman had only come to the reservation a few months prior and was not a major player in events leading up to what happened. http://en.wikipedia.org/wiki/Charles_Eastman

John and Mary had never met. They were like two hummingbirds who had also never met.

Yeah. I looked him up. This man saw so much in his lifetime. He doesn't 15 years living in nature as a native American. He witnessed, first hand, the hostilities between the US government and his people. He was then basically assimilated into American life and became a doctor. He loved long enough to see human flight, WWI and the rise of the automobile. He died just before WWII broke out. That man had plenty of stories to tell.