This is a very fun idea, and I'm intrigued by your synopsis. However, the musical utterly obliterated Cosette's character to make an audience of sexually unsatisfied teenagers root for poor, forgotten Eponine. Let me set a few things straight:
1. Cosette was described by Victor Hugo, the godly author of Les Miserables, as not only selfless and grateful, but adventurous and courageous. She did not wrong Eponine, nor would she ever.
2. The child Eponine was a spoiled tattle-tail and certainly no friend of the child Cosette. Neither was she a friend of Marius's, but rather was described as a stalker whom Marius continually had to avoid. While Eponine is indeed to be pitied, she should never be deified.
3. Eponine's intention in going to the barricade wasn't to save Marius. She, like Marius, assumed that everyone at the barricades would die. If she couldn't have Marius, she wanted to at least make sure Cosette wouldn't get him either. She hoped both she and Marius would be fatally wounded. The only reason she stole Marius's bullet: she wanted to die first.
I realise this is fanfiction, so of course I can't expect accuracy, but the characters should remain consistent as they are the reason people read fanfiction.
There are so many things the musical botched, when copying Victor Hugo's plotline, characters, and even moral and political lessons. I amicably suggest you read Les Miserables and continue writing in the future as you seem, if uninformed, to be a creative person with good ideas.
Lauren Mockett: Reviewer