Sentient

Of course all your concerns are valid, and I'm not suggesting there are not problems.
Did Chief Sealth make such statements? We would only have physical proof if he had knowledge of the written language (which I do not know if he did.). or there was audio recordings.
Chief Sealth was made famous for finding a way to help his people live. He was a leader. Proof: His people do live. They thrive.
He should be remembered as a leader that strove to have his people survive.
As far as 7 generations, if you care when or how it started that's your business. It is a real saying that real people use today for the purpose of having one stop and think of your actions. Walk carefully and think of those that you will be affecting.

And 7 is Spiritual perfection.( 1+6=7 ) 1 being the first to be thanked the Creator of all (Center). Followed by thanking the grandfathers of the 6 directions. E S W N Sky & Earth. This is a common practice among many.
Can't say who started it and please don't say the hippies.