Here is the real reason why Justineau is white...
there is a difference between a book, which ignites intangible imagination, and a film, which is very visual and realistic.
can you imagine the uproar, at the ending scene, when one lone black woman, essentially the last of humankind, is enslaved by a demonic pale white kid with blue eyes? that is a very powerful visual, that can incite a lot of unnecessary attention about racism, slavery, etc.
i think the filmmakers made a good call on this one. the black girl wins, which is a popular consensus these days anyway, and the levity of the original story is not lost.
it's not exactly a win-win, but i think the other option could be so much worse. especially for folks who did not read the book, and just decided to judge the film as is.
we live in a day and age where everyone is so easily offended. look at hermione's race (the stage play), or rue's (hunger games) causing so much attention.