I'm not suggesting anything.
I have no access to City accounts and i don't know what has been done or not done behind close doors. What i think is FFP was designed to stop City and PSG and changes were implemented along the line to make those clubs fail. I also think City and PSG used tricks to go around, find loopholes because they are ambitious and won't settle for being middle of the pack clubs. Now, both clubs used different methods. PSG invented a new type of contract "nation branding" and argued that it was worth hundreds of millions because there were no equivalent, thus no fair value market. I don't think anyone can argue it wasn't an attempt to blindsight the rules. In my opinion, what City has done according the hacked emails is way smarter. The only problem is that those emails have been made public.
As you said "My understanding of the emails was that UEFA where arguing Etihad paid a low amount and Abu Dhabi United topped it up", UEFA is considering the money was coming from your owner through those sponsorships. This, de facto, makes them related parties. Thus, UEFA is taking the numbers directly given by the sponsor as written in the email as the new market value. Scummy move, especially applied retroactively, while City can't balance the books anymore by selling a player.
My whole viewpoint on that case is that UEFA was pressed to trap the new (arab) money clubs by the established elite and those emails gave them the ammunition to fire a shot at City. However, i don't think City is the only club to do that. Pretty sure that there must be some backdoor deals when Jeep or other firms from the Agnelli decide to sponsor Juventus or when Audi, Adidas sponsor Bayern while being stakeholders. And, i have zero doubt the Emir of Qatar has been pressing those Qatar firms to sponsor PSG. Just that UEFA doesn't have emails of the same nature listing how the payments will be made. These emails are the basis of their argument that you have been lying and not cooperating.
As stated in this tweet, i think the difficult task for City is not the problem of the fair value of their sponsors (they can say the value was accepted by UEFA in 2014) but the fact they are accused of untruthful reporting of their financial situation. And this is prolly on this axis UEFA will try to justify their sanction.