I think several different issues are being conflated here.
Nobody has suggested that William or Kate personally read this letter, still less that they were responsible for its contents or handling. The point is that the Telegraph reports it was passed to the joint Kensington Palace office serving both the Cambridges and the Sussexes. That is a relevant detail when considering the potential circulation of the correspondence within the Royal Household. Dismissing the reporting as bollocks doesn't establish that it is wrong.
Baital makes a useful point about the practical difficulties of overseas police enquiries. Clearly, information can be incomplete, investigations can go nowhere and the UK police cannot compel another jurisdiction to act. But that is a separate question from whether the Met followed the correct procedures in assessing and referring the information it received.
And that's precisely why the IOPC investigation matters. It should establish what the Met did, what it failed to do and why. We shouldn't prejudge its findings.
As for the suggestion that Meghan secretly acquired a copy for future use, or that Harry and Meghan were somehow complicit through silence, those are theories, not established facts.
I'm not particularly interested in allocating blame to William, Harry or Meghan before the evidence is available. I'm interested in the documentary trail, the decisions taken and who was informed.
Surely that's a more productive discussion than inventing conversations between Harry and Meghan or declaring that someone is 'cooked' before an independent investigation has even concluded.