The people studying these tribes assume these utensils are things the "uncontacted" tribes trade for with other tribes who get them from normal civilization. I went and looked it up. These people aren't stupid. They know that these are man-made things, and not magical.

I don't know what kind of diseases these people have or don't have. I suspect you can create a remarkably healthy group of people by isolating them for a thousand years, and letting generation after generation of weak, immune-compromised, deformed, and genetically-flawed people die horribly out of the gene pool.

As for the "unspoiled tribe" SF story, it's been beaten to death. I actually made a couple of hundred bucks in college doing spaceship designs for someone who was going to do a movie based on Lloyd Biggle's book Monument. I have comic books like that, there are short stories like that, and I think Avatar comes vaguely to mind.

As for being given a choice? Who knows? Like I said, everyone who lives in a nice apartment in a city anywhere on the planet had an ancestor who lived in a squalid hut somewhere. You'd be surprised how appealing civilization can be to people who haven't gotten so used to it that they feel nothing but contempt for the luxury that generations fought to achieve.