Abuse of monopoly position to force their payment system use.
The argument of being a Monopoly is for the courts to decide.
Maybe.
Any such case would take years to adjudicate.
Apple isn't going to just lie down and accept it. They will appeal it all the way to the highest court. Look how long the Microsoft antitrust case took.
What would happen is that the case will outlast a presidential administration. That's what happened with Microsoft, the Clinton administration started the case and when Bush came in, they backed way off the case.
So it depends on how friendly the company is to the politicians in charge.
The EU could also impose fines. But if say the EU said Apple has to allow side loading, they could abandon that market, because their market share in EU countries are in the single digits everywhere but the UK, which will no longer be a part of the EU.
It would be tougher for Google, because Android has huge market share in those countries.
Actually I didn't follow Fortnite that closely but didn't they start out side loading on Android and then eventually moved to the Google Play Store? So why did they change? They didn't have to pay Google a fee but then moved into the GPS?