About Napoleon Hill, although people often attribute that Ivy Lee story to him, and although he seems indeed the most likely origin of the story, I have not been able to verify it with a reliable source.
For example, one of the many people attributing the Ivy Lee story to Napoleon Hill is Dave Albano, who says about it: "this is the same technique handed down to me personally by my friend and mentor Bob Proctor, the very same one handed down to him by Earl Nightingale, who had it handed down to him by the legendary Napoleon Hill... the Father of Personal Development." It's true that Bob Proctor worked some years for Earl Nightingale and surely knew the story from him, but about the rest of Albano's phrase, also in this case I have not found a reliable source to verify it. Albano also, maybe confused, mentions Andrew Carnegie -as well known by Hill- instead of Charles M. Schwab. Perhaps people who knew Nightingale, like Bob Proctor, if asked, could confirm the Napoleon Hill -> Earl Nightingale part of this transmission chain.
The oldest verified source that I know including the Charles M. Schwab & Ivy Lee story is Earl Nightingale's audio betseller Lead the Field (1960), published and well-known during the life of Napoleon Hill (1883-1970), who knew both Schwab and Lee. However, Nightingale just explains the story and does not mention its origin in that recording. By the way, differently to most other versions such as Bob Proctor's, Nightingale does not mention Schwab, just "the president of a steel company" and Ivy Lee.
In 1960, Nightingale -apart from his popular radio program "Our Changing World"- also published a condensed audio version of Hill's "Think and Grow Rich", so the two men, both famous (see an Earl Nightingale's bio), probably cooperated about that time (also not confirmed).
Napoleon Hill was a lecturer as well, and it's possible that the story passed from Hill to Nightingale through a public lecture, or a personal conversation. So, again, this is not sure, just what most probably happened.