With a pre-1900 POD have a surviving native American state inhabit an area controlled by OTL United States
Why on earth would they do that? They've never done such a thing before.During the period when the 'Oregon Country' is jointly controlled by the UK and the USA, the UK signs a treaty with one of the local tribes that recognises the tribe as a sovereign nation.
Again, the UK wouldn't spend political capital to do that. It's not in their interest.When the rest of O.C. is divided between the two powers the UK insists that as a part of the deal the USA must itself recognise that state..
Moreover. Even if the Brits DID require such assurance, it would be pro forma and the US would immediately ignore it.Maybe it's actually formally a British protectorate until after WW2...