It looks like you've very well traced some of her heritage back to the British Isles. Oftentimes, a presumed American Indian/Native ancestor turns out to be Scottish, Irish, or Welsh. These people frequently have straight dark hair, and either have a slightly dark complexion or end up tanning as they do farm work in the more southern parts of North America. Over time, the story passed down that "Granny was as tan as an Indian" becomes "Granny was an Indian."
Depending on which area of the US you think her Indian heritage may have originated, a paper trail may be sparse. Since civil and church records for whites are much more common, your quickest start would be to prove or disprove British Isles heritage for her ancestors who you are unsure about.