I don't see what is far-fetched. Sarah pushing Karen down the stairs is perfectly consistent with the publicly alluded-to theory of Borce disposing of Karen up along the Calder. If Borce disposes of Karen's body, it does not mean he killed her- that then leaves open the prospect of Sarah doing it, accidentally, and Borce covering for his daughter. Further, Anthony Rickard claims Karen molested him. Possibly Karen said to Sarah she will be offering tens of thousands of dollars for Anthony's rehab, or other purchases, as a way to placate Anthony into not speaking out publicly about it. Sarah could have first heard of the molestation then and there and "went ballistic", to use a phrase Sarah has used elsewhere, and pushed Karen down the stairs. I don't see why you're against this theory, Mrs G Norris. I didn't even raise it first, Puggle did; though i am inclined to find it credible. Obviously it could be incorrect, in the end, which doesn't mean that it now, at this preliminary stage, is to be considered incredible.
Further, others have noted Sarah's demeanour at the infamous 'did you kill your wife' press conference was odd. There could be something to that, although face-reading "experts" here on websleuths are carrying it a bit far, for my liking, and although initially dismissing it as hogwash by myself, i am now rethinking matters about the Sarah stair-fall concept.