> That's why everyoneisover18.com forwards the request to my bank or my broker

Doesn't work. The response won't be signed by real-broker.com.

The permission request/response itself goes direct from the server at restricted-site.com to the server at real-broker.com over TLS, so you can't MITM it, it's not controlled by the client and you won't be able to just pass out a cached response.

Your malicious client plugin could potentially forward the client session details to you, so you could operate the broker page, then log in to your bank's portal and approve that request, but I don't think that's going to scale very well and I imagine your bank is likely going to rate limit you.