I feel that the night's events were as a result of a horrible argument gone wrong, however I do believe that GBC was planning to 'get rid' of Allison - just later down the track. So LollyPop, your thoughts about whether he could be traced back to being in the area prior to that terrible night are very interesting. As you said, maybe he had explored the area days or weeks before.
I also agree with you about the Crossing and I have my doubts that this would have been a place that GBC checked out to dispose of Allison's body or chose as the drop off point on the actual night. It would have been a massive risk to attempt to park a car in the long, bushy grass at night - not knowing if the ground would drop or if there were any obstacles hidden in front of him. It can also be quite a boggy area - from first hand experience! IMO if GBC explored places prior, they would have been areas that were more hidden, such as up Ugly Creek a bit, off Wirrabarra Rd. This is also where I feel he took her on that awful night.
My sister has a CX diesel Captiva and it's a 4WD (all wheel drive I think) but it's not a very high 4WD. I couldn't see my Brother-in-Law confidently driving it through long grass, near banks and creeks in the dark. Then again, maybe GBC was really stupid and loved a risk.
Anyway, thanks for your thoughts LollyPop. It's interesting to hear everyone's ideas and opinions

Can't wait for some new info too - it's been a long time waiting.