This is the crux of it for me.
Did someone in the UAE (i.e., HH) top up the sponsorship funds? Maybe, maybe not. The emails seem to suggest they did.
But if that money was invested in each of the sponsors and then paid into City's accounts directly from Etihad, Etisalat, Aabar, etc. (as also suggested by the emails), I don't see how UEFA have much of a case.
It's not like they'll have any access to those companies' accounts/bank records as they have no jurisdiction over them. And in the very unlikely scenario they even asked to see them, they will have been told to fuck off.
So, all that matters is that if City stated X million as Aabar sponsorship revenue, then City's own bank records need to show that same X million arriving directly from Aabar. And the leaked emails suggest they were very careful about ensuring that happened.
Now obviously, if City's accounts show 25% of that X million came from Aabar and 75% came from elsewhere (i.e., ADUG, Sheikh Mansour), then we're bang to rights. But we surely wouldn't be appealing if it was that cut and dried.
So, my own hypothesis is that UEFA "know" we have circumvented FFP; the emails show it. But they have no other proof. And even if those emails were admissible, they could theoretically be explained away as meaning something else.
Rather than allowing us to get away with what they see as blatant "cheating", and knowing that the evidence they have won't stand up in CAS (or court), they have sought to inflict massive reputational damage in the hope that some of that damage will stick even when the punishment is overturned.
This ties in with the rumours that UEFA were desperate to agree a deal with City that saw us accept guilt with a token slap on the wrist. That suggests they weren't at all confident that their "evidence" would stand up to intense scrutiny.
When City told them they weren't interested, UEFA went for the only other option left to them — nuclear reputational damage. This explains why the punishment is so excessive; they know it won't be upheld, but they had to go in hard for the world to take notice and make at least some of the mud stick.
That's my theory anyway!