They arrived in this continent before anybody else. Thus, They are entitled to sovereignty. Moreover, The US signed treaties with them, Which means that the US recognized them as nations. Yet these treaties were later flagrantly violated and the US Army (the Yankee Army really) massacred them. The least that the US should do is recognize and respect their sovereignty. That is if we have any decency left as a nation.
We have taken so much from the Natives that we owe it to them to give a little back, and we can do this first by declaring their territory sovereign and letting them govern themselves as they so choose, and we should financially support these nations whenever it is necessary or possible.
They should be given the opportunity, yes. After everything that they have been put through and the things they have had to put up with it's the least we, as a nation, can do. It may be too little too late but any amount of respect is better then none.
Native American Nations are somewhat sovereign and somewhat not. It honestly doesn't matter much because these pieces of land are virtually useless and so far out and away from society that they are barely sustainable, if you could even say they are viable in the first place. Native Americans deserve better than what they've received, but there doesn't seem to be much talk of improving their situation.
To be completely sovereign would be a disaster for Native American nations. Remember, in many if not all cases, we forced them onto reservations with very little in the way of actual resources. They would not be able to support themselves. I think the current arrangement where they receive subsidies from the United States federal government, and limited sovereignty, is adequate.