I do think Jennifer Lawrence would do a great job, but I would be confident in Carrie Henn being up to the task, even in a very extended role. I'm of the mind that actors shouldn't be overly elevated or placed on a pedestal. They're not brain surgeons. They don't save people. Their entire job is to pretend to be someone else and be convincing of that. With some acting lessons, Carrie Henn COULD do this very effectively. She does come across as an intelligent, genuine human being, and I bet she could pull it off.
Now the question is, would she sell tickets? From sheer star-power? No. But, this would be a great human story, particularly because she was out of the lime light for so long, and one could argue that this would be even more compelling. Picture the headlines: CHILD STAR TURNED TEACHER RETURNS FOR ROLE OF A LIFETIME. It really could just work.