It depends on one's perspective. Some people believe them wholeheartedly because they are based on personal experiences that are deeply emotional and life - changing for those who claim to have had them. They see the sincerity in the storytellers' eyes and believe that there are things in the world that science can't yet explain.
The significance could be to fill in gaps in the story. By going back, we can add details that were missed the first time around. For example, if it's a story about a person's life journey, going back in the story can show how early experiences shaped later ones.
One 'I went to heaven and back story' could be about a person who had a near - death experience during a serious illness. They might describe seeing a bright light and feeling an overwhelming sense of peace. Another could be someone who was in a terrible accident and while unconscious, felt like they were floating upwards towards a beautiful, warm place filled with love. And there are also stories where people claim to have met deceased loved ones in this 'heavenly' experience.
Well, it depends on a lot of factors. If the reasons for the initial split were superficial and both have grown and changed for the better, it might have a happy ending. But if the core problems weren't addressed, it could just be a repeat of the past with more heartache.
One possibility could be Luffy changing key events to prevent the losses of his dear friends. This would add a lot of drama and emotional depth to the story.
Well, those who say they've been there and back often describe a sense of overwhelming love and light. But of course, these accounts are highly subjective and can vary greatly from person to person.