I think that Australia would have gotten bored of Cat within the next few weeks unless she received a really favourable edit. Being a divorcee, if her edit was made to look like she was the sweet woman who had her heart destroyed she may have truly garnered the sympathy votes. Plus she's a nice girl, people would favour her over villains like Katie.
In regards to Lawson, I think he would've gone pretty soon. So many people are saying Clawson brought him down, but I think the endearing Lawson from the first week or two was an act (at least somewhat). He snapped at Skye really badly a few times, ways which would have made him lose supporters regardless of Clawson. I think Lawson would have eventually snapped at someone that he was getting annoyed at. I got the feeling he was not good with confrontation and any confrontation would make him look like a horrible person, regardless of the subject matter.
So no, I think neither would have made it to the finale. But I think that Cat would have actually lasted longer, despite that not being a mainstream thought here.
