Yeah, I suspect that Helen and Ella are the same person.
I don't have any info on Nellie's father apart from his name was Lindsey Crawford and he was a Maori. And that info was only from this page http://www.territorystories.nt.gov.au/handle/10070/227683
so I don't have a verified source.
Darwin Library is a bit far away for me, but if you are able to recall the book titles and authors I might be able to source a copy locally.