What about a slightly different scenario... just throwing it out there.
Just to be clear before I begin, I'm not victim-bashing here at all, just brainstorming...I feel huge amounts of empathy for Karlie but it does seem possible that she was caught up in a difficult situation of some variety.
What if Karlie was doing the runs from state to state (as a mule not a one-person operation), perhaps even only the once.
What if DH and HP really were just on a general family holiday, caught up with K&K in Darwin, continued on & headed down towards Adelaide.
Had the accident. HP flown to Alice. Karlie back in Alice, hears about the accident (visits HP & DH & offers to help out, not forgetting she knows the kids also and would have been pretty shocked).
Perhaps upon hearing that HP needs to head to Adelaide for rehab offers to help pack up their house and bring some belongings to Adelaide on the next run. Perhaps Karlie confides in DH what she is doing. Perhaps the thought of the money &/or drugs gets the better of DH and he decides to take it for himself (and HP/child). If he makes it look like K&K have gone on the run with it, he gets the money and offers to take over the role. Perhaps stealing her identity initially is more about hiding her murder from the syndicate bosses so he could keep the money and have a future income (if only for a little while) as well as anyone who might be looking for her.
With HP still in hospital he racks up the kms to finish the jobs that Karlie was supposed to.
Little Khandles is an obvious liability to him... he either takes on the role of her demise himself or passes her on to someone else (is this where suitcase man features?).
Have no idea exactly how HP's role plays out in this scenario (yes to the fraud at a minimum), but I think she features less as a main player in the period between the accident and K&K's death than a lot of others seem to be crediting her with.
I haven't got the time/brainpower to iron out all the creases with this scenario myself, but it keeps popping back up again and again so I have to throw it out there. Hope I haven't overstepped any lines here regarding what can/can't be discussed.
Exhausting just trying to get my head around all of the convolutions in this tragedy... seems to be at least a few month long saga full of "unfortunate circumstances" for all the major players, that ultimately led to the death of these two angels.