I reject out of hand the idea that she should deconvert in the closet and systematically lie to everyone she knows.

I had to do this until I was able to sever myself from parental support at age 20. It certainly wasn't pleasant and sometimes I still have nightmares about being discovered breaking the Sabbath (though I've told my parents long since). But if you ask me whether I would have rather remained religious, TEN THOUSAND TIMES NO!

I don't know your parents, but I know the people who will be Wednesday's. Nothing terrible will happen to Wednesday if she deconverts: she would make her parents a little sad, and they would probably try to argue her around, but they would not do her harm or kick her out of the house or otherwise mistreat her in any way, shape, or form. I do not object to deception in self-defense (or defense of others in Jews-in-the-attic-in-Nazi-Germany situations), but Wednesday will not require deceptive self-defense.