Answer 1: With the mock-up, it is much easier to discuss needs with the client before you start to write any actual code. Changes to the mock-up are really easy and fast to do, compared to modifications with real frontend source code.

Answer 2: There is lot of suitable applications available to do mock-ups easily. You can also use paper and pencil to create mock-up.

Answer 3: You can modify the security configuration class to allow access to all endpoints without authentication.