The Old Man and the Sea is a novella which is about exactly what the title says; just add a big fish, and that's the essence of the storyline or plot. What happens between the old man, the sea, and the fish, though, is an epic battle. The old man fights valiantly against the forces of nature (the sea, his "brother" the fish, and the sharks) as well as against his own mind and body. In the end, he catches the fish but destroys both it and himself in doing so. He's not defeated, but he has suffered a great loss. That understanding is what we would miss if our narrator had not told this story from an omniscient point of view. Take out what is in Santiago's heart and head, things which are revealed by such a narrator, and we would simply be watching an old man battle a giant fish. It's true we would have heard him talk to himself aloud and could perhaps have understood this respect and struggle a bit; instead, we understand that Santiago sees this fish as his true brother. We would see him fish, but we would not understand his respect for the fish or the sea. Without that, we really wouldn't care very much whether he won or lost this battle.
One other key element we would not have been privy to without an omniscient narrator is his dreams. It's these dreams which show us his loves, his passions, his degeneration, over the years, into an old man. Without this kind of "inside knowledge" we get from an all-knowing narrator, Santiago is more of a pitiful figure than a kind of tragic hero.
In short, then, think of all the things we wouldn't know if we didn't have an omniscient narrator to tell us. Look at your list and determine why any or all of those things are important to understanding and appreciating this story, then decide if the story would have been better or worse if Hemingway had made a different choice.