I would use
Annabel and call her
Bella if it seemed to suit. I don't think it's too contrived.
I'm under the impression that a lot of Isabellas are called
Bella, and I tend to assume that someone called
Bella is an
Isabella. I agree that
Bella as a stand-alone name does seem too much like the word, but if you don't know whether it's a nick or not, the word doesn't count that much.