I cant see the tin, but until about 1890 or so, non assimilated Indian tribes were considered sovereign nations by the U.S. . The Lakota could not "seceed" because they were not part of the United States. Also, the Metis are less of a nation and more of a culture group.
The Lakota will get crushed. The Metis have a slightly better chance of succeeding, but their prospects aren't great either.
Forty years later, and the Lakota and Indian allies / vassals are fielding the equivelant to brigades. The USA has just gone through the civil war and the public wont support a war that involves the use of conscripts.
The USA would have the ability to match it, but after the exhausting civil war, they may not have the will to match it - at least not immediatlyEven if they did have that kind of power, the USA will easily match it. .
I agree, the best a northeren plains Indian / metis nation could hope for would be to enter the United States as a native american majority state.America saw that land as theirs and nothing was going to stop it.