Third party app stores don’t play into the legal liability of developing for iOS without a license. Chances are Apple still has every right to charge a percentage of revenue as the licensing fee for usage of iOS APIs.

Any regulation that requires allowing third-party stores, would be an anti-monopolistic move. So it would almost certainly include a requirement that those third-party stores be allowed to process payments themselves without paying royalties.

That’s not how copyright and patent licensing works. If it did, the computing industry would look very different.

Copyright and patents aren't some kind of deep magic that binds the whole Earth. They're just laws. Legislators are free to change how those laws work...