My latest theory is :
allison comes home on the Thursday - an argument ensues - perhaps about money, relationships, behaviour at the school cross country anything. The argument turns violent and Allison runs from the house and jumps in her car and she drives off, followed by her husband. In the ensuing hours he convinces her via mobile phone to talk with him - the meeting place is near the roundabout at Moggill and Kenmore Rds. More like;y he would get into her car, the argument continues and struggle, fight takes Allison life. We now two cars and one driver. GBC calls someone (Dad or girlfriend, an unnone third person) and they meet him near the roundabout. The third person and GBC drive one of the family cars home (the one Gerard was using) perhaps they drove the wrong way round the roundabout (it is late at night, no one is around, they are panicing) hence why the police are testing driving times. GBC and mystery person drive back to ABC, in GBC car, in her car near the roundabout. They then park mystery persons car and move Allison body, as she probably was in the drivers seat, into the boot and travel and dump her body. This would all take some time, I think you would become more sensible and restrained as the time wore on and start think of covering up what has happened. Very early on the Friday morning they return to the other car and once again return to BC home at Brookfield. They sit and talk for some time to get the story straight and the 3rd person leaves. 7.30 has been decided as the time to call police, so GBC waits until this time and calls the missing person report, thinking that this will buy them a number of days before any suspicion is aroused.
The mobile phone was used to talk or message between the Baden Clays, it needed to be got rid of so he threw it away in bush near his home, not incriminating the roundabout area and lead people to believe that something a happened to Allison around the local area, perhaps attacked on her morning walk, when the mobile phone became such a vital clue the phone was retrieved and got rid of knows where.
I think when the police took such a great interest in the case very early the whole plan started to unravel, and the BC are trying desperately to stay one step ahead. The kiss and the road accident were ways to divert attention and to engender sympathy and to paint themselves as a very loving couple.
When I rewatched the video of GBC and his sister this morning, I was struck as to how modest, by Kenmore standards, the parents home is.