For the first part they have https://openrouter.ai/docs/guides/overview/auth/oauth (which I haven't used). That gives you an openrouter key for that user (openrouter bills the user).

What value are you providing that you'd take some further cut? You could take the money from the user via stripe and then pay your own openrouter bill instead.

I'm still amazed at Apple's 30%, it's just so brazenly high (like: "fuck you, we're Apple, we do what we want and you'll lick it up"). Unfortunately, too, other companies follow Apple's lead, and so this massive rent seeking percentage has become normalised.