The one fact that we do know about the Etihad sponsorhip (and about Etihad in general) is that the Executive Council had been covering it and Etihad appears to have been the recipient of substantial state aid. It's there in the Open Skies Case documents.
We don't know for sure who Pearce was referring to when he used 'HH' so it's nonsense to say that it was "clearly" Sheikh Mansour. If it was referring to Sheikh Mansour, rather than MBZ, why not use 'ADUG' instead? They appear to use that in a number of other emails and we must assume that Pearce, as a senior adviser to the Executive Council, would know the correct protocol.
I'd very much doubt that who HH refers to be our key line of defence though, or even part of it. As you've been saying all along, if we have a contract with Etihad and can show that the contract has been fulfilled, then there's no case for us to answer. Again, I don't agree with that line of reasoning. The source of funds is certainly the core of UEFA's case here, as the Der Spiegel stories demonstrated.