Re: Imagine that...
The judge's job is to make sure the jury only get relevant evidence and to then give them direction on how to assess that in relation to the charges. In my experience of jury service (done it tiwce now) defenece barristers seem to throw a lot of irrelevant questions at witnesses, simply in order to confuse the issue - or overwhelm the jury with so much information it confuses them.
I was only doing crap cases though, nothing worse than GBH with intent - so we weren't getting the highest calibre of people. But the only defence barrister we had who managed a proper bit of oration in his final speech was the one who asked the least questions of the witnesses. And kept it relevant.
Judges are very careful about witnesses giving their opinions from the box. It's the jury's job to come to those kind of conclusions. So if you're quizzing someone about their state of mind, you can have opinions, as in "I thought he was going to attack me with the knife so I threw a chair at him". There the opinion is relevant.
But you can't say, "he went behind the screen to hide the drugs" - because unless you saw him do that, you're giving an opinion. You can only say I saw him go beind the screen, and we later found drugs hidden there, and he didn't have any on him when we searched him. It's then up to other witnesses and evidence to prove he had the drugs and up to the jury to find whether he in fact did.
So in the same way the prosecution can't just make stuff up, or assume stuff, neither can the defence. You can't ask a witness what they believe is the interpretation of the facts, except in limited circumstances. For example when you're examining why a witness acted in a certain way, then their interpretation of facts is relevant.
An expert witness, for example, is also allowed to draw conclusions - becuase they're supposed to be witnesses for the court rather than prosecution or defence. It's only in unusual circumstances that defence will seek their own expert witness. Although I think this is different in the US system.
Justice needs to be seen to be done and that means allowing leeway for dubious evidence to be presented and confirmed or exposed, especially if it comes from the investigators.
That's all very well. But the jury aren't lawyers. It's frustrating, I've served on a jury and felt I didn't have all the evidence I'd have liked. But part of that is that it's impossible to gather all the evidence, and almost everything is subjective. But there are also rules to try and make the system fairer, such as not always revealing previous convictions. And protecting the defence by not allowing dubious evidence to be put.
The point about rules of evidence are that things that don't fall within those rules, by definition, aren't evidence. And the opinions of the investigating officers are definitely not evidence, and would be far too dangerous to give to the jury. Given that the whole point of a jury of non-experts is to keep the system honest. We pay judges to rule on what the jury are allowed to see, lawyers to try and organise the case in favour of other side, jurors to weigh the evidence to establish our best guess of the facts, and judges to help the jury organise those facts and tell them what is relevant to the law. I recommend jury service to everyone, as it gives you a much clearer view of how things work.
If the defence think the previous opinion of the investigators was correct, they can put forward the same fact the investigators had, and see if the jury agree. Or agree with the investigators that the subsequent evidence they uncovered pointed in a different direction.