City have two ways of attacking an adverse verdict and any punishment UEFA might impose. The first is itself two pronged: they do not challenge the validity of FFP as yet but appeal to CAS that UEFA did not carry out an investigation which conformed to the process UEFA itself has laid down and anyway the evidence, when considered in its entirety, shows that the rules were not broken at all. If this fails City can take the court to the ECJ and argue that FFP is not enforceable because it is a flagrant violation of the right to invest. There are other grounds on which to challenge FFP but this seems the clearest and most obvious.
The FC Sion case did not raise any such question of a fundamental breach of European law. The Bosman case had already established the principle that the football authorities were subject to the law just as other bodies are. Certainly FIFA behaved with arrogance and intimidation in the FC Sion case but they have been served a timely reminder since that the law has a very long arm and Mr Blatter and his colleagues are rather humbler these days. And should City secure a judgement that FFP is contrary to the law any punishment would be invalid and a claim for commercial damage could cost the football authorities a literal fortune. If City showed that FFP was the work of a cartel which also abrogated to itself the enforcement of its own regulations much of the "football establishment"/"football royalty" would be ruined.
I don't know how the adjudicatory commission will respond, because Ceferin appears to represent the minor, especially central and eastern European, leagues who wish to restrain the financial dominance of the major leagues and so see FFP as essential (though inadequate!) but they have to be very careful and realise that they probably have to be on rock solid ground every step of the way. Pandering to a cartel could be disastrous.