Bella is a lovely name in its own right. Of course it can also be a nn for
Isabella,
Annabella, etc.
I have no problem with it also being a nn for
Annabel since nns are not scientifically bound by the formal name. Nns (to me) are the place to show affection, whimsy, creativity. Let your imaginaiton run wild with nns. They are as free as formal names are regimented. Personally I can see BuggiewoogiebugleboyofcompanyG as a nn for
Annabel. So using
Bella as the nn doesn't seem strange at all.
I like using
Annabel (since that seems to be your favorite) as the formal name because (as you mention) it gives her options. So I vote for
Annabel, nn
Bella (or Dewdrop or Standswithfist or Anbo or Tulabug or . . . . )