Yes, you do have the option to submit paper immigration and customs forms. The cabin attendants on my inbound flight handed out blank cards en route.

To my knowledge (although I’m happy to be corrected on this point), pre-registration isn’t strictly required for the quarantine checks either. I believe (though can’t speak of this first-hand!) that you can present proof of vaccination/testing upon arrival, although the process will take longer than if you have a blue screen and QR code.

Cheers.