countess the reason Nevill couldn't have made the call upstairs is that the line had been disabled by taking the kitchen phone off the hook.
The belief is that he was badly injured upstairs and the billet casings support this.
He went downstairs to try the phone down there, had a physical altercation with Jeremy/someone there before the final four shots were fired into him.
Apple from what I can see the physical evidence showed a mixture of blood in the silencer which came from Sheila Caffell plus likely Nevill and June Bamber. Some marker in Sheila's blood indicated its presence there. Sheila wasn't tall enough to have shot herself with the silencer on so you have either
Sheila shooting herself once and then taking the silencer off to put it away in a bag in the cupboard OR
Someone else wielding the gun and staging a suicide scenario before realising that it would be obvious Sheila couldn't have killed herself with the silencer on. This person then hid the silencer back in the cupboard.
Both Sheila and Jeremy's fingerprints were on the gun but Sheila's hands were too clean and her nails too pristine to support the murder/suicide theory.
Any other potential physical evidence was lost with how badly the crime scene was managed.
Largely Jeremy was convicted on circumstantial evidence and the testimony of many other people.
I think the defence really went for Ann Eaton when she gave evidence and rightly so as she had the most to gain with Jeremy's conviction.