My thoughts exactly.
And maybe it was Borce that said that Sarah saw Karen, too, before Sarah left that morning. But maybe Sarah didn't see Karen at all. Maybe Sarah assumed Karen was still tucked up in bed, sleeping, when she left. Whereas Karen may have been laying there, strangled or suffocated.
When Sarah learned what Borce had said, she may have clammed up. She may not have wanted to go against her father's lies, as uncertainty hit her.
That may have been the turning point in Sarah's pleas for her mother's return, her 3AW interviews, her speaking to the media.
(As we can see by your immediately-above post #222, Borce doesn't mind speaking for Sarah ..... “If we sit down you can actually go and ask my daughter as well,” he said.)