I think the fancy lawyer has made it difficult for them. Hope Starry sleeps well at night.
So, say Borce has killed Karen at home, bundled her body into the car and is heading up the Calder. At some point, he thinks F^%K, the phones, and stops to deal with them somewhere around Blackhill Rd, maybe to turn off and dispose of Karen's and turn his own off, or whatever. Then he drives her to Macedon and returns to Avondale Heights to wait for her to not come home from her "walk to clear her head". He is aware that he has stuffed up with the phones, so why doesn't he weave his Calder jaunt into the narrative for the police at this stage? If he knows enough to deal with the phones during the disposal journey, he knows what the phones can reveal. He can bust out his drive story straightaway and not look guilty bringing it up later. Maybe the stress causes this big error.
I only ever used a short stretch of the Calder to go to Watergardens, getting off and going up the Melton Hwy, until late last year when I discovered that Kings Rd exit/entry, so I never really considered the possibility that they would have traveled up a fair bit of the Calder from AH to get to the shopping centre. So this makes me wonder if Borce and Karen used to travel to Bella Bleu to work together and if they did, whose car they took. Did they set out to go to work together and something erupted?
I also never paid much attention to the talk of their connections to property near where the initial searches took place. Did something draw them there? Or perhaps the drama with AR - was the $850 to shut him up, whether he is lying or not, one of them thinks the easiest thing to do is just give him money, and this turned into a heated fight. Maybe they are together for a purpose, Karen alive, where the trackable data stopped, the data that caused the various searches. And then Borce didn't think about covering up that they were there, only from then on once it went bad. Later he sets the last known place she was back in AH, forgetting to account for the evidence of that time they spent together between AH and Blackhill Rd.
Anyway, it's all speculation when it is strung together on very few facts.