If it is all so patently obvious as you seem to think perhaps you could explain how initially the police surgeon, all officers, bar 1, and the pathologist thought they were dealing with 4 murders and a suicide for over a month.
About a month after the murders a DCI new to the case was appointed to review all the evidence and concluded SC was responsible.
The case turned on two highly contentious issues: Julie Mugford's testimony and a silencer.
Forensic and pathological evidence is not the panacea that TV shows such as CSI and Silent Witness have lead us to believe and cases are rarely solved in the first few weeks. One thing common to those TV shows though is that the protagonists often learn the hard way about making assumptions instead of following the evidence.
As you rightly point out, the physical evidence did not initially ring any alarm bells, Sheila
could have battered her already injured father, swinging the rifle over her head like a club as evidenced by the broken lampshade and she
could have shot herself twice but Taff Jones and others were blinkered to other possibilities by the assumptions made at the very beginning.
It's disingenuous to say that the case turned on only two issues and ignore the emerging pattern of evidence over the course of that month and beyond -
Dr Vanesis said he would have expressed concerns about the clean state of SC had he been called to the scene.
The complete lack of any forensic evidence on Sheila, no blood, wax on her nightie or lead on her hands from supposedly loading the cartridges, no cuts on her feet despite there being broken glass all over the kitchen. (No sugar either)
The admissions made by JB himself after JM had grassed him up regarding entering WHF via the windows, the burglary ridiculously passed off as "testing security" and sneaking a peak at the wills in the safe.
The hacksaw blade that was found to match scratch marks on the window catch.
JB breaking in to WHF after his arrest to get his car documents - or pretending he did to explain any forensic evidence that he didn't know had not yet been found.
The bike at Goldhanger that had mud on the sides of the tyres.
The parts of his wetsuit found at WHF
The evidence of JMs flatmates that cast doubt on the timing of the call to her and thus JBs account of NBs supposed call and when he called the police, which he already contradicted himself with between interviews.
The mysterious lack of towels in his bathroom - who doesn't leave a hand towel in the bathroom? JBs explanation that he kept them in the airing cupboard lacks sense -bath towels yes, but who would go to the toilet, wash their hands and then go to the airing cupboard to dry them? ( I think the washing machine had been on too but not sure )
Bamber beratting PC West and asking what was taking so long, "my father sounded terrified" - directly contradicting his own tardy actions.
Bamber standing outside telling police about all the guns in the house and that SC was familiar with them all but later admitting he had never seen her fire one as an adult.
Stan Jones overhearing JB laughing on the morning of the murders that set him on his trail.
JM was not the only witness to give evidence of JBs expressed dislike of his family either - he would bitch about them to anyone that would listen.
Once the police started to look at JB with suspicion, things changed until the conclusion EP had been on the wrong track couldn't be ignored.