With respect to University of North Dakota, there are about 9 different tribes in North Dakota which are part of the historic "Sioux Nation." Of these, 2 raised objections to the University calling themselves the Fighting Sioux. Concurrently, NCAA policy prohibited use of Native names, unless approval was obtained from the Native Tribes in question.
In spite of efforts of University Athletic Dept, no arrangement was able to be reached between UND and the 2 tribes in question, and thus, to continue in good standing with the NCAA, UND has rebranded itself, the "Fighting Hawks."
It is under the exception above, (Agreement with the local tribes) that Florida State continues to use the Seminoles name. I'm not clear on those details, but the Seminole Nation appears to enjoy its relationship with FSU, and thus, the NCAA allows that nickname on the basis of this logic, "If the tribes like the use of the name, it is not insensitive, racist, or abusive."
Of course, these things exist in the realm of public opinion and public narrative, which affects NCAA policy, and are not in any way 'legal' in the sense of state or federal statutes.
Thus in the case of SeattleNHL using a nickname connected to the Native American Nations, there is no legal precedent. The matter would simply be a question of how the school of public opinion would react. And, it's my personal belief that it wouldn't be worth the risk, especially when the area is perhaps inclined to be sensitive about such things anyway.