Is the unexplained, unexpected sudden death of a woman in her early 50s not suspicious? Especially when her partner failed to ring 999 in a timely way or attempt resuscitation etc? Genuinely curious. Because if it is only suspicious once the cause of death is confirmed after post mortem, then potential crime scene evidence is by then long gone surely?
Add into that some background info about the partner, trouble in the home...
The facts remain that she was murdered and the initial police response and subsequent investigation failed to uncover evidence of this at the time. The evidence must have existed, as the crime was committed. The potential failings of the post mortem process are a linked, but separate issue.
I know someone who died unexpectedly in her sleep (undiagnosed heart condition). The police were involved initially, and although they were sensitive to her husband, who had made considerable efforts to attempt resuscitation and was extremely distraught, the potential for foul play was clearly one thing to be eliminated until they knew more. This did not, of course, mean arresting him immediately, but also did not mean doing nothing to secure potential evidence in case something later came to light. Oh, and he had no history of violence either.