Stefan has made the same point. The PL's evidence was clearly challenged by the club, that much is clearly stated in the judgment, as are the tribunal's reasons for accepting the PL's evidence. It seems however as though the club did not deploy every argument they might have: assuming that's right, they (the club) obviously had their own reasons for holding back on the challenge to the motivation. I wouldn't like to speculate on what they may have been - it might have been something as simple as a lack of time before the panel - but it does seem clear that they could have gone harder, further.
On your questions, there is a distinction between "void" and "voidable." The essential difference is that the latter is valid until set aside, the former is invalid from the inception. Whether a thing is void or merely voidable depends on what it is you have an issue with, and why.
In this case, it seems to me that the position is this. The tribunal found that the APT rules constituted a restriction on competition "by object." In competition law something can lawful for one of two reasons: "by object", namely the restriction was intended to have a particular effect, or "by effect", namely the restriction wasn't particularly designed to have that effect but the law of unintended consequences is at work. A good analogy exists in anti-discrimination law - for instance a regulation that is specifically directed a particular groups - women, LGBTQ+, BAME etc - would be directly discriminatory, that is discriminatory "by object" whereas something that was directed at (for instance) childcare provision would be discriminatory "by effect" because in practice childcare provision tends to be undertaken far more by women than by men.
In City's case, the ruling of the tribunal was that the APT rules were a restriction "by object." They were intended to have precisely the effect they did, and that effect was contrary to our competition law precisely because they allowed one form of owner subsidy and prohibited another. I personally don't see how rules that were intended to have an unlawful effect can be anything other than null and void from the get-go. So the answer to the first question is, IMHO FWIW, a very good one.
By contrast, I don't think there is any real prospect of challenging FFP. The reason the APT rules were found to be unlawful is not because the object of regulating the transactions between clubs playing in the PL and associated parties is unlawful in itself- neither was the previous regime which regulated transactions between clubs and related parties for that matter. But the way in which those rules were framed contravened the principle that if you are going to regulate in that manner, you don't do it in a way that favours some clubs but not others. I don't think FFP could be subjected to the same criticism.
