The Cosette problem is only there in most stage/film incarnations because its played that a passive, inexperienced, but beautiful and genteel Cosette, and a manly, pretty, but gentle Marius is just what each wants - and its therefore love at first sight. It can just work if they can play the first look scene right. Its far stronger with a more assertive Cosette out to move on and get her man - but only one recent stage Cosette has been able to play that.
This is were I think it worked in the film as opposed to the Michael Ball clips I have seen. I might be wrong but it looks like Michael Ball just played it as a wet drip (Which may have been how he was directed to play it) but for me, Eddie nailed it in the 'love at first sight' role.