First time poster so be gentle,with me! - have been following this with interest now for some time. Hopefully the below hypothetical theories are allowed here - if not please let me know and apologies in advance if this is not allowed.
Given the the behaviour of the family, the financial pressure, the argument the night before etc, is there a possibility what we are dealing with here is a suicide followed by a cover up?
if after the argument With BR and SR, Karen decided to take her own life. The next morning BR would quickly realise this means no life insurance, and will also bring shame on the family. BR could have convinced SR to stage a disappearance and then got VR involved to help with the logistics.
This type of scenario is the only way I can see SR being knowingly involved, and regardless, even if it was murder, LE will have to work to prove it over a hypothetical scenario such as the above, which wouldn't be too easy to do.