Funny how people are questioning the reason for BKA investigating CB over MM. Isn't that their duty once someone passes them the evidence?
SY attained the HB confession in 2017, but despite MM being British, they have zero jurisdiction in this case. The crime took place in Portugal and the suspect is German. The evidence HB gave them, while being compelling, is clearly not enough to raise a charge on and therefore clearly not enough to extradite him to the UK to be questioned or charged. So it makes perfect sense they appeal to their German counterparts to see what they know (and can do). And to be clear, we don't actually know what conversations have taken place between the various agencies but I suspect those conversations are what led to BKA taking the lead here.
When this information was given to SY in 2017, CB was himself the subject of an European Arrest Warrant to be extradited back to Germany to face charges of sexually abusing his former partner's little daughter. He was finally detained following the playground incident (another paedophile act) in Portugal in 2017 and jailed for 15 months. So when this all kicked off, CB was being dealt with by the German authorities. They had him in jail for the abuse, and were in the process of building the drug case against him for the Sylt shenanigans before he did a flit in 2018 and got extradited again from Milan to face the charges in Germany.
As for why PJ have not just been passed this information, given the crime took place in Portugal, so that they can investigate themselves...? I think that's pretty obvious. They have absolutely no incentive to prove CB is guilty of anything. This case has caused immeasurable damage to their image and their tourism industry, they simply want it to be forgotten about. The same question could also be asked about the DM rape case. It happened in Portugal and the Portuguese were the ones who had the hair that convicted him, by the same token, BKA could have just passed on the HB and MS accounts about the rape videos and left them deal with it. They didn't. They applied for a EAW and charged him in Germany to be tried in court. IMO, the main reason is that neither BKA or SY have much faith in PJ, I don't want to come across as xenophobic but I just don't think their attitude to police work is on the same level as the British or the Germans. Nor is their motivation when it comes to the MM case. It's made especially worse due to HB's claim that he actually told the Portuguese authorities about the CB confession (which is what kickstarted this whole investigation) back in 2008, and they did absolutely nothing about it.
Amaral spoke about CB (although not mentioned by name, it was clear he knew who he was) well before this case became public knowledge. How does a retired and disgraced Investigator attain this knowledge? IMO, it's because the PJ is like an old boy's club and quite frankly, leakier than the sources close to the "McCann's". They simply can't be trusted to investigate this case impartially or fairly. And, that's probably one of the biggest contention points in this case and why there is such bad blood between the different agencies.