I thought it was pretty open & shut initially, I really did, but after reading some of the ideas shared here in earlier threads, I've realised there are at least a couple of scenarios where the impersonator may have been completely unaware that Karlie & Khandalyce where dead.
If that person had been a friend of Karlie's, privy to the reasons she'd left Alice & any issues she may have had with her family. AND that person had just lost 2 kids in horrific circumstances & endured a leg amputation & 3 month hospital stay? - I could see her being overwhelmed & distracted enough to not think much beyond her own pain if she got home & was told her friend Karlie had moved away.
If she started getting texts from "Karlie" - "Centrelink are hassling me, I can't get there myself, the car broke down, Khandles is sick, everything's a mess. They're going to cut me off if I don't turn up - I wouldn't ask if I wasn't desperate H", please?.
& the calls to family : "I can't deal with talking to my mum yet, you know how bad it was when I left, can't you just ring her to get her off my back for a while? Please?"
I expect she'd have been treated with morphine etc after her release from hospital, add that to what she'd been through already (ie not JUST losing 2 kids, but being trapped with their bodies, a younger injured child & her own injuries, possibly for hours) & she wouldn't have been thinking too straight.
I don' t think it would've been too hard for an "interested party" to turn the screws & add a little more pressure - "What kind of friend are you H? Your mate's in trouble & you won't help her out?"
There are a lot of possbilities here, not all of them point to the impersonator/s being guilty of, or even aware of murder.