Your review of Smart's story gave me a lot to think about, and thank you! I've long suspected that Den has altered a story that seems a "tall tale" in the first place, and that Den's version depreciates Smart because Den didn't like him.
Den's memories of Olivia are vivid, and take up much of the book. To me, it's always seemed like this was the part of his life he remembers most fondly. Den has more in common with her than with his parents, and she includes him, begrudgingly or not, in a number of adventures that he remembers for the rest of his life. I believe that Den's parents took Den back only because of Olivia's marriage, and that Den knew this, and blamed Smart for it. Freed by her engagement from her financial dependency on Den's father, Olivia sent John some early twentieth century version of "Dude, getting married over here. Your kid though, seriously". Lacking air travel, email etc. this exchange, and the Weer's eventual return, would have taken a long time, and Den would have stayed with Olivia in the interim.
There are other comments. later in the book, that suggest Den and Julius were not close. Den's version of the Mr Tilly story has to be colored by his opinion of Smart. But what did he change?