I kind of like the idea of and AU in which Les Mis is actually written by Marius. Like as he got older he just had an increasing amount of Feelings About Things and started obsessively researching his mysterious heroic father-in-law, as well as everybody else connected to him and Cosette.
So he traveled around the country interviewing people, looking through documents, checking locations etc. and finally wrote down his own interpretation of the events based on the things he found out or remembered. And because he’s Marius he couldn’t help but address various other things he’d become interested in (like sewers, argot, etc.) and some things he used to be interested in (Napoleon, Waterloo). He also became much more republican and much more conscious of social problems later in life so the book became increasingly about those things too.
… So I guess everything in Les Mis would be from an older Marius’s point of view in this universe. Honestly I think it might explain a lot.