I'm not a lawyer, so my opinion means nothing but I would suspect the answer to your first question is no. In fact, I can't actually see a way that apple could be forced to lower their cut (by governmental means). AFAIK, the government doesn't typically mandate the costs of consumer goods and services. I think the only thing they could be forced to do would be to open up the iOS platform to developers who don't wish to use the App store. I.e allow other app stores onto the platform or other payment methods. The only way that would then result in them lowering their cut was if there was an exodus of developers to one of these new stores, and Apple needed to make their more attractive. You can see how this may have been possible if that was how iOS had worked from the very start, but considering how Titanic the App store is, I think any newcomers would just be crushed. Perhaps, with the exception of Epic who essentially just want an App store for fortnite and fortnite alone.
As for evidence, you're unlikely to find it as those internal costs are not made public. However, based on the size and how hands-off the store is, I would guess it maybe costs a few million a year to keep it all up. According to this article Apple says that last year they were able to take a cut from $61 Billion worth of transactions out of an overall $519 billion of overall revenue (The other stuff comes from ads, physical purchase like uber etc.) so at a rough esitmate -> 61* 0.3 = $18 Billion last year for Apple.
At those levels, you don't need to know the actual internals because there's no way the maintenance of that infrastructure costs close to 18 billion dollars a year.
