Vendor MIME types (for API versioning)

As I have decided to follow the jsonapi.org standard, I decided to do just what your example shows.

Regarding your questions:

1. Your MIME type should be registered
2. I believe that each version and type should be registered, as they are unique types
3. x- prefix has been deprecated as noted in a comment
4. application/json-v2 would not be valid
5. You could return 415 Unsupported Media Type or simply 400 Bad Request