No, I don't believe they could. The courts in common law jurisdictions generally don't rule on speculative harm, they require an active controversy to rule on.
The larger topic of which this is a part is something called "standing." Lack of standing is quite a common reason for a court to deny hearing a case. Not only does there have to be a harm, the party suing has to be the party that suffered the harm. So you also cannot sue on behalf of others.
This is the reason for the initial actions Epic took. Tim Sweeney has been chomping at the bit for this sort of suit for years. But they needed a harm to use as the locus for a suit. There has certainly been a big PR campaign surrounding it to try to sway public opinion to Epic's side, but the action itself was necessary.
Just because Epic stands to make an extra buck or two doesn't mean that their incentives are not aligned with small developers and consumers in this particular moment.
Probably not. Part of the reason Apple has prevailed in the past is the lack of direct consumer arm from their actions. By offering the discounted rate it's no longer disputable that Apple's actions harm the consumer.
Epic could have taken the matter directly to the courts and taken action (whatever it was) after a verdict. Heck, they could even have tried to make a case to the European Union, which seems to have a knack for pursuing this kind of thing. A ruling over there wouldn't directly impact the rest of the world, but it could get the ball rolling.
Instead, they decided to make a spectacle of this whole issue to try to rally customers to their cause. For that, I have no sympathy. Customers are being used as cannon fodder here. And it's not by Apple.
I agree that the App store cut is too high. But this is not the way to go about things.