Nell is a decent enough name to stand on its own. I'd expect a person to be named
Petronella if I'd meet someone who said it was their nickname. I don't like it for the ones you suggested bc I don't think it makes sense to stuff an N in front of a Hel-/El- name to make
Nell. (same way I don't really understand why I don't really get '
Ned' for
Edward or '
Peggy' for
Margaret) But then again, they do at least have an -n- in them later on... I think
Ellen is too short to have
Nell as a nickname. Of your choices I'd prefer it for
Eleanor.
Annabel would work too or
Marinella,
Noella,
Ornella,
Penelope.
Cornelia would work as well.
Penelope "
Nell" would be nice actually. I don't like "
Penny" all that much.