Another common theme could be self - discovery. For both Jane and Billy, the impending parenthood might make them look at themselves in a new light. They may discover new strengths or weaknesses. Struggles with responsibility are also often seen. With a baby on the way, they have to deal with financial, emotional, and practical responsibilities, which can be a major part of the fanfiction's plot. For example, Billy might have to take on a new job to support the family, or Jane might struggle with balancing her career dreams and motherhood.
I'm not sure of specific 'jane and billy pregnant fanfiction' stories off the top of my head. But you could try searching on fanfiction platforms like Archive of Our Own. They have a wide variety of fanfics, and you can use the search filters to look for ones related to Jane and Billy with the pregnancy theme.
Barefoot Billy in the 'barefoot billy true story' might be a unique individual. He could be a person who rejected the norms of wearing shoes. Maybe he had a philosophical reason for it, like believing that being barefoot kept him more in touch with the real world. Or perhaps he grew up in a situation where shoes were a luxury he couldn't afford, and over time, it became a part of his identity.
Sure, Billy Bathgate is fiction. It falls within the realm of creative writing, where the author weaves a tale that may or may not have any resemblance to reality.
Jane is likely a fan - created character. She could be an ordinary girl who somehow gets transported into the Naruto universe and starts writing a diary about her new life there.