1) More details would be needed about the apps being consumed through the hub. But I would suspect that some of the apps will not work correctly with external users in anonymous mode through the Power Apps portal. Its possible, but not all apps are going to be work in that kind of context.
2) Yes, that should be possible using a Power Apps portal. Again its hard to be certain without a lot more details.
3) Custom connectors to use custom APIs will still require a premium license, so I don't think this will help with the licensing.