Maybe I'm a boring pragmatic but wouldn't it be more useful for these organizations to band together and assist the Washington Redskins with the name change? I'm sure they could raise sufficient funds to help the owner with some of the necessary costs associated with this change. They could also assist with a Washington DC area Campaign to rename the team and find out what other names would be acceptable to Redskin fans.
When a highly successful person feels like he is getting pushed around he isn't going to submit to any of your demands even if they are reasonable and good. This could be worked out if one side was willing to swallow their pride with the understanding that the other side will never swallow their pride. That's how you get things done. You get more flies with honey than with vinegar.
Since neither the National Congress of American Indians, the United South and Eastern Tribes, the Affiliated Tribes of Northwest Indians, the Cherokee Nation of Oklahoma, the Comanche Nation of Oklahoma, the Fort Peck Tribal Executive Board, the Grand Traverse Band of Ottawa and Chippewa Indians, the Great Lakes Inter-Tribal Council, the Gun Lake Band of Potawatomi Indians, the Inter-Tribal Council of the Five Civilized Tribes, the Inter Tribal Council of Arizona, the Juaneno Band of Mission Indians, the Little River Band of Ottawa Indians, the Menominee Tribe of Indians, the Oneida Tribe of Indians of Wisconsin, the Oneida Indian Nation, the Poarch Band of Creek Indians, the Seminole Nation of Oklahoma, the Sault St. Marie Tribe of Chippewa Indians, the Standing Rock Sioux Tribe, the Confederated Tribes of the Colville Reservation, the Three Affiliated Tribes of the Fort Berthold Indian Reservation or the United Indian Nations of Oklahoma have any legal justification for their case I would suggest they be the side that compromises in order to get this accomplished. The owner won't be bullied around. If you bully him more he'll be even less inclined to reason. If you bully him a little bit more than that he'll be even less likely to listen to reasonable request. If you bully him more than that he could just quit listening altogether and nothing will ever get accomplished.
This can only be accomplished if one side is willing to chose action over winning. Winning feels good but it should be about more than just winning. The name probably does need to be changed but you have to be willing to do what it takes to make that happen.