I know they happened like I know anything that is true that I have not personally experienced, by an overwhelming set of evidence from people and sources that I have grown to trust because they have been generally correct in the past
And "Gell-Mann Amnesia" is just an over-drametic way of saying that the news has errors, that you notice them in areas you understand, and you nevertheless trust the other areas
And I agree that such trust is generally not warranted. People need to read media with a sceptical mindset
Media has errors and biases, but they really fall into two classes:
(1) In their choice of what to cover they omit facts that do not fit their narrative or world view
(2) They ascribe motives to people's actions, without evidence
But neither of the above two classes of errors apply to the six facts of Prigozhin's life, which you listed. They all happened. Maybe other things happened in his life that are also relevant. Maybe the motives ascribed by others to why those things happened are wrong. But those six things nevertheless happened.
See the References section of the Wikipedia page as a demonstration of the evidence for the six facts: