The error in the 1990 Census was corrected in 2000, which is good. However, to continue to complain about the omission in 1990 serves no purpose: it is not possible to turn the clock back. I admit that, as an outsider from the UK, I find it difficult to understand your arguments. It is not as if the people you defend are pure breed American Indians; for all I know there may be a handful of those still around, but inter-breeding has diluted the American Indian blood to the extent that surely most of those who choose to add their ancestry in the 2000 Census as Americans first and American Indians a very distant second.
As for claiming tribal lands for those with American Indian ancestry, that is like inhabitants in the UK who can trace their ancestry back to countries in mainland Europe claiming land there: crazy.
My hobby is heraldry, which involves tracing family coats of arms and the origins of family names. In my research I come across many family names that originate in one country and move to another, and migration is as common now as it was when America was populated by immigrants. The result of immigration is that a country’s population changes. In the UK, the large immigration of people from the Caribbean in the 1950s and since then from Africa and the Indian sub-continent has radically changed Britain. At no time have I noticed a movement by the indigenous Brits to reclaim areas now lived in by immigrants.
Be proud of your ancestry by all means, but live in the present, not in the past.