It does seem that a large swath of the Les Mis fandom doesn't care thaaaat much about canon, but I think most people consider the Brick (affectionate fandom name for Hugo's doorstopper) to be Canon Supreme. Tumblr allowed for people who didn't have the time or inclination to read the entire Brick the chance to absorb snippets and quotes and analysis from Brick fans. The minor characters we obsess over really aren't even in most of the alternate versions, so even if the aesthetic or plot or characterization of more major characters is taken from other versions, the existence of many characters has to come from the Brick, so that's where anyone who writes those characters into their works has to go back to eventually.

I've likened it to people who write Batman fic in the Batman v Superman setting but pull their minor characters from DC Comics, or people who write BBC Sherlock fic but include Victor Trevor.

On two-teenagers-in-a-trench-coat: I turned 18 riiiight around the time I deleted my LJ and took a break from fandom, so I sidestepped having to take responsibility for my participation in 18+ stuff pretty neatly. I don't think I ever saw fallout like the fallout you describe – makes sense, though. A hard line to walk.