Perhaps the uncertain walls are a symbol of the changing relationships within the city. They could represent how the people in the city interact with each other, with the walls acting as a metaphor for the barriers or connections between individuals or groups that are constantly evolving.
Rayleigh would be a great candidate. He's an experienced pirate and has already trained Luffy for a while. He has a lot of knowledge about the world of pirates and could give Luffy a talk about the deeper secrets of the Grand Line or how to deal with the powerful enemies that lie ahead.
It could be. There have been some strange and unexplained events in history where people have discovered evidence that someone might have been living secretly within the structure of a building. For example, finding food stashes or makeshift beds in wall cavities. But it's also possible that some of these stories get exaggerated over time, so it's hard to say for sure if there's a completely true 'Man Living in the Walls' story in the purest sense.
The Wall from 'A Song of Ice and Fire' is not only a physical barrier but also a place full of stories and a line between the civilized and the wild. In 'The Wizard of Oz', the Emerald City wall gives an air of exclusivity and wonder. As for Mordor's wall in 'The Lord of the Rings', it is described as menacing and a significant part of the overall defense of Sauron's domain.
The 'the girl in the walls story' could be about a girl who is trapped in the walls, either physically or metaphorically. Maybe she's a victim of some sort of abuse or a strange supernatural occurrence that confines her to the walls. It could also be a story of mystery where the girl in the walls has a secret that the main characters are trying to uncover.
If we consider this from a more symbolic perspective, perhaps 'sin' here doesn't refer to the religious concept in a traditional sense. It could be a name or a metaphor for something negative that 'climbs' or infiltrates the purity of the temple, like corruption or bad intentions. However, without a clear backstory, it remains quite ambiguous.