Bridget, you are really digging a deeper hole for yourself now. A reasonable ground can be an inference which may be drawn from the surrounding circumstances. In this case just dealing with the silencer evidence: i) JB denied being responsible for the murders therefore the defence case was that Sheila had to be responsible, ii) the rifle was discovered by police without the silencer fitted, iii) the silencer was subsequently said to have been found by a relative days later in a box in the cupboard under the stairs, iii) the silencer was removed from WHF, examined and handled by several of the relatives and retained by them for several days, iv) the FSS found blood inside the silencer which was either Sheila's or, less likely, a mixture of Nevill's and June's, v) items of Sheila' bloodstained underwear were removed by a relative from WHF, vi) although possible, the suggestion that Shela had used the rifle with the silencer fitted initially then removed it, placed it in the box in the cupboard and then shot herself, was an unlikely scenario, vi) the only other explanation for the presence of the blood inside the silencer was contamination, either accidental or deliberate.
Against that background Rivlin would have been perfectly entitled to raise the suggestion of contamination, even deliberate contamination, because that was an inference which might be drawn from the evidence. I would go further and say that if Jeremy had insisted Rivlin would have been under an obligation to put such allegations to witnesses and he would have been in breach of the Code of Conduct if he had refused to do so.
There was a high profile case in the 1970s, an IRA bomb trial at the Old Bailey. The head of my chambers was appearing for one of the accused. The evidence against him was that his fingerprints had been discovered on a timer found at the address used by the bombers. The defendant's instructions were i) that he was not involved in the bombing, ii) that he had never seen the timer and had certainly not touched it and had never been at the address where the timer was found. There was no doubt that the fingerprints were a perfect match for the defendant. In the light of those instructions the only basis upon which defence counsel could challenge the prosecution case was by directly alleging that the police had planted the figerprints. He demonstrated that it was possible to take a lift of a finerprint on a glass using sellotape, then deposit it on another surface. The deendant was not surprisingly convicted but counsel had done what he was obliged to deo. The trial judge was the notorious Mr Justice Melford Stephenson. He was of a similar view to Bridget in that he claimed that defence counsel who had pursued this defence had done so without reasonable grounds. He directed that their fees be cut and he reported them to the Bar Counsel. Defence counsel appealed the fees order and sought a ruling from the Bar Counsel. The Bar Counsel robustly supported the position of defence counsel, in a blistering criticism of the judge. The fees reduction was also overturned.
The above example is of a case where the evidential basis for the defence presented was far lower than existed in JB's case in relation to the possibilty of contamination of the silencer.
I do know what I am talking about Bridget, trust me! ;D