I don't believe the tribal councils at Standing Rock or Spirit Lake are ever going to endorse the name. The polls on Native American opinion on this issue are really unreliable. Native mascots are not a big priority given all the other things they have to worry about, so when asked in a casual poll they are relatively indifferent. But, when Indian people focus on the issue and learn more about the specifics, their opposition gets very strong. This is a fairly common phenomena in all democratic politics.
Trying to convince the tribes will be a big waste of time. They made up their minds a long time ago, and their opposition will only get stronger the more attention they pay to it.