I don't think this is the way the plot will go, but what if she really is Elizabeth? ie, she's trying to escape an oppressive older brother and forced marriage by running away, then pretending to be someone else when she was caught? The maid would have to be in on it as the only person who could testify she wasn't Elizabeth. And the apparent personality difference would be her finally dropping her forced smile.
It will be fun looking for clues, so I hope this gets continued.