I've had some thoughts after reading this thread over the past six weeks. This is what I've come up with. I'd be interested to read any constructive criticism. If any dates or places are incorrect, please let me know.
Karlie may have already supplied documents to Centrelink verifying her identity before she was killed. The person posing as Karlie wouldn't need to show them at an interview. All other transactions and contact can be done by phone or on the internet. In any case, Karlie would probably be carrying identity documents with her if she was going to be working whilst away from home in Alice Springs.
Maybe Karlie met up with her killer in Adelaide; even on the promise of working for them. After her murder, the KP Drilling Company may have been set up by Karlie's killer, using her identity documents, in order to apply for ever increasing loans from the credit union or other sources (maybe that's where the deposits due to fraud come in). Much easier to get a loan if you are a small business, after making even small, regular deposits, than it is if you receive Centrelink benefits. The multiple ATM withdrawals that were made in NT (and elsewhere) could have been connected with a person who has a truck-mounted (or towed) portable rig used to drill for bore water.
Maybe the stop in Mount Gambier was to pick someone else up on the way to Canberra, possibly the person who lived in Charnwood and who was known to Karlie and/or her killer. Or just to visit family living there.
Karlie was probably killed sometime just before Christmas in 2008, around the time she withdrew money from the ATM in Charnwood. Either she went willingly with her killer toward Sydney and was killed in Belanglo State Forest or she could have been killed shortly before (maybe even at the Charnwood house.) As to the killer's motives for killing her at that time, I have no clue.
As for little Khandalyce, maybe she was left in Canberra whilst Karlie went on her final trip. The killer returned and took Khandalyce back to Adelaide, with or without an excuse as to why Karlie hadn't returned with them. There are many possibilities as to why Khandalyce may have been murdered. She may have been considered of no use to her mother's killer, her murder was due to violent abuse or to control someone else with her remains being used as a reminder of their own tenuous grip on life. Maybe Khandalyce's remains were moved once the Angel's killer was incarcerated or the other party finally managed to escape. Or the killer got rid of Khandalyce's remains when they moved away or her mother's remains were found (ie, there was a risk that the jig would be up).
I wonder if this person has preyed upon other young women before Karlie, with or without children such as Khandalyce, and this time just saw a chance to take advantage of them even after their deaths at their hand. A very disturbed individual, not to mention the people who were accessories.
Sorry this post is so long, the cogs in my brain turn slowly.