First and foremost by not cordoning off WHF as a crime scene area thus allowing all and sundry troop in, whether it was a murder/suicide or not initially, it was still a crime scene. Then there was the matter of the lack of temperatures taken to make a note of the time of death as again, initially, could have avoided what we have 36 years later.
By all and sundry I assume you mean police officers who needed to carry out critical roles: firearms, senior officers, police surgeon, scene of crime officers.
I understand a small pet dog was placed in a wardrobe to preserve the crime scene.
There's no evidence anyone was alive when the police and Bamber arrived but even if I consider the possibility Sheila died some time after the other victims I do not believe any methods of time of death exist where you can set your watch by it or anything remotely like.