There is no simple proof. Only tiresome and perplexing proof that the general public don't have the attention span for. The only "simple proof" that the media could take interest in, would be a confession on her part. That's not going to happen IMO.
The diaries become part of the video narrative, the dialogue is there for the actors to use. This video also shows how Nevill was in the bedroom and JB shot June first then shot two rear shots to Neville as he swivelled from bed, and how Nevill lowered his head forward to get the trajectory to his mouth. The video shows a one armed Nevill fighting under the mantle piece and reconstructs the scratches being delivered to the underside in that even fashion.
The video shows Sheila in a drowsy state lying down to be shot.
Then it shows him exit by the kitchen window, and pulling the latch down with the Myster nylon cord. It shows him cycling home and getting rid of all the forensic evidence.
This is not difficult to make, but not a living soul will believe what they watch. There will be an uproar.