Personally, I've not thought so much about the mark I will make when when I leave this earth. That puts so much pressure on people to perform. I take the example of Nelson Mandela, who lived serving his people. He never thought that he would be a symbol of justice and freedom.
I do not care so much about being remembered. As your post revealed, no one is remembered for so long. I've read about the Beatles but never heard any of their songs. But at some point this boy-band existed and they were a worldwide sensation. They simple just did what they loved doing--making good music, without paying so much attention as to leaving a legacy, which they did.
Like your post explains, it's not all about being remembered but having an impact that out lives your lifespan. That can be accomplished by doing even the littliest of things.
How can you know? At some point he likely recognised his own influence and that will have affected his future action from that point. He was not a blind man, he aid attention so it is unlikely that he missed it.
Really?
This is what everyone should be doing.