You misunderstand me. First off, that unit is very clearly native american inspired. It's hardly "interpretation" that it's a tomahawk. It is far more clearly native american than the brute, the only other barbarian unique, is european. The only real european aspect of him is the horned helmet. Considering this other unit, it's not so bad, but it's still putting a bad vibe out there to say that native americans, more than every other group except germanic tribes, are barbarian. If they were truly adding barbarian variance, that would be fine, but instead it is effectively singling them out from the crowd, which isn't really a step forward.
As for the Shoshone, they are a step forwards. Except for how they've been implemented - everything about them in game is how they have existed in relation to the US. They are not the Shoshone that should be in, they are the "injuns".
Also, your last point is highly contentious and really opinionated. There are a host of good candidates for civs, many of which native american. I personally wouldn't have put the Shoshone amongst them, precisely because they were always going to have an americanised version their history representing them.