Look at where Sanders actually lived—an area with a lot of Native people. How many Pretendians come from places like that? How many become powwow dancers?
And should it turn out that he was not Cherokee, it wouldn’t affect my essay. No one was attacking Sanders for being a Pretendian. They were attacking him for failing to adopt the values of liberal members of the bourgeoisie. You are the first to question his background.
Now, you can argue all you want about the absence of evidence. I am firmly on the side presuming innocence.
Further, if you want to accuse Sanders of faking his identity, you should at least talk to any surviving friends and relatives. Your research may have taken you down the wrong path. And if it did not, you still should confirm what you found before assuming it is conclusive. That’s journalism 101.