The couriermail article says that the kids were dropped of at a "friends" home, so I think we can rule out that they were dropped off at GBC's parents house on this information. the link has been provided previously.
I think this is much simpler than whats been suggested.
No kids at home would more than likely allow an argument to get out of hand. Now for the neighbours to hear it, I'd say that they were outside, possibly allison running/walking out of the home and getting into one of the cars....
Maybe she didnt go for a walk, but planned to go for a drive, to let off some steam.
she drives her usual route out to moggill, sayt his is down brookfield road, stops in the car park at the roundabout to gather herself, or could have been followed by GBC, or to text some friends, etc. GBC turns up in white car, they argue in the carpark, someone calls the argument into police. They both drive off together in one car, whether willingly or forecably, leaving the other car behind.
The thing the police could be looking for with regards to the roundabout is someone being dropped off to pick up the other car. I'd say one of their cars were parked at those shops.
This is where the lady that has been interviewed comes in. I doubt she would be called to assist with the dumping of the body, but probably called to assist with returning one of the vehicles, and maybe help to clean the vehicles, home etc. If GBC was in a panic, he'd call the one person close to him that isnt a family member that he has some control over.
The police turn up after having been called earlier about a DV incident at the roundabout involving a silver crv and a while 4wd. They have a light bulb moment, Gerard says he went to bed at 10pm but the call about the DV was made at about 11pm, instantly contraditing his story...they send out the search party, get the warrant becuase of his lies and the DV call.
How did they get the warrant? They use the call to the police about the DV, the matching vehicles at the residence, and GBC's contradicting story about the time he went to bed. Enough to suggest that he may get rid of evidence at the address, enough to get the warrant.
The lady that has been interviewed has probably hired a lawyer as she plans to play the "I didnt know what was going on, he jsut asked to help me return a car to his house" card.
thoughts?
Pretty sure this is my 3rd theory but I feel that I am close here.