Doesn't "hon vill bli ensam" work here?
That sounds a bit weird to a native like me. She wants to be alone, not become alone.
mmm I think I get it
It would work in a sentence like 'Hon vill bli lämnad ensam' (She wants to be left alone).
Shades of Greta Garbo?
Why 'att vara' doesn't fit here?
Modal auxiliaries like vill never take an att when they're followed by an infinitive. See this post: https://www.duolingo.com/comment/7075383