If Pocahontas’s grandmother had Cherokee DNA, then that can also be tested with the DNA of all the descendants of that grandmother and all the descendants of that grandmother’s siblings.
This is true, but do note that even siblings of the same mother and father can have a different %age of a geographical gene group. Let’s say one parent came from Africa. One child can have, say, 45% west African, and the sibling could have 38% west African. It’s not an exact thing because children of a parent don’t get the same exact genes from each parent.
True, but if anyone of that grandmother's (or her siblings') descendants have no Cherokee blood than Warren's grandmother didn't have any and Warren has none.