Okay I understand your position. You have invoked simple modal logic.
One of your premises appears to be that Sheila could not do this crime and remain unsullied. This premise is false, so you should use hard science as requested to prove Nevill did not phone Jeremy.
Obviously this crime can be replicated using actors to show how Sheila could pick up a loaded gun, go upstairs and shoot her mother, shoot her father in the mouth as he came upstairs to investigate, then when he is incapacitated swing the gun by the barrel to bludgeon him, reload the cartridge with her fingernails remote from the cartridge, and shoot again.
Then we can have her lying down and shooting herself twice, after realising the first shot missed her brain.
If necessary she can wash herself before her suicide.
Samson, you asked me to apply 'hard science' to prove the telephone call never took place. Refer to my earlier post, I agree you can't because it was not possible in 1985.
This is the point I am repeatedly trying to make to you sir, and for the final time. The significance of the telephone call is that even if he told the truth about the call, it excluded any possibility of 3rd party involvement and so painted himself into a corner.
Thus by his own admission the killer could only be Jeremy or Sheila. Now recall the mountain of physical, forensic and circumstantial evidence that confirms that Sheila could not possibly have done it. Case closed.
As the gentleman above mentioned the rest of your post is not worth a sensible response.