This is a novel by one of Australia's most gifted and famous authors. There is a degree of critism here in the literary world regarding his choice of the title, The True History, when in fact its a novel. The worry is that future readers will think it's a biography.
I don't think Kelly ever tried to get the story of his life published but he did try to have certain events put to print.
The word "adjectival" does not crack a feature in the Penguin book of Australian Slang and I've never heard it used in the manner that Carey does. That's not to say it didn't have some currency over a hundred years ago but I would be surprised if so.
As far as I'm aware Ned's child is a figment of the novelist's mind.