TL:DR; would be nice if messages could be returned when customer checkout information triggers validation warnings etc.
At this time, the shipping address is sent as payload data to Apps, but if you validate it as fraudulent or incomplete there is nothing you can return back to the customer to inform them. In the case of fraud I guess you could return either an insane shipping amount, or nothing, but for more general errors, you cannot inform the customer of anything wrong.
You completely misunderstand the issue. You only get the customer's shipping address in Checkout. And the only way to inject an App to work with that data, is in Custom Carrier Shipping calls.
If you read my explanation I made ZERO mention of rates as being important. Strictly looking at address validation here.
What does your question We just need the Shopify interface built mean?
Your App use case is super limited here, and since you don't seem to like that, you need to be more open about what you really want to expose other possibilities if any.