absolutely.
From an investigative viewpoint also;
Sometimes you know the individual is guilty as hell. But there’s a difference between “intelligence” and “evidence”
even when you find evidence, it has to be properly handled or else it becomes inadmissible - you’d be shocked at how draconian this can be. for example, in her youth Hillary Clinton managed to get a non-guilty verdict for a guy who’d had sex with a child. The child’s underwear had been cut up - the semen/dna piece had been kept and served as evidence. The leftover bits were lost. Hillary successful argued that the chain of evidence therefore, had been broken. the DNA/Semen sample wasn’t presented in court as a result, so the jury never heard about it’s existence. It was a clear cut case, The man had sex with an underage girl, but the evidence wasn’t handled in accordance with draconian rules and thereby because pretty much irrelevant - it’s an example of how the law really is an ass.
even when you have evidence, and youve handled it properly; it has to be agreed in advance of any trial and the prosecuting body has to agree it is sufficient to progress - due to the high court costs this isn’t any easy feat.
I often hear people say that juries “hear all the evidence” but actually, they usually only hear a proportion thats been agreed in advance. Evidence that is very damning sometimes doesn’t get heard at all - because it might bias the jury.
another thing that is misunderstood about criminal trials - they don’t just seek to get a guilty/non guilty verdict. They must also be “safe verdicts” aka not one that can be appealed later down the line - and again, people would be truly shocked at the mundane things people successfully appeal over.
so in summary - the Portuguese police might’ve known something no one else does, hence their conviction and assuredness in pursuing the McCanns. Or they might have just decided to go down the easiest, seemingly most obvious route.
logically, I would concur that the McCanns could absolutely have been responsible, even accidentally, and there could’ve been a wealth of information in existence that isn’t in the public sphere.
intuitively though, I don’t think they are involved.